diff --git a/apps/web/src/components/allocations/AllocationModal.tsx b/apps/web/src/components/allocations/AllocationModal.tsx
index fa5c3ce..0e9a015 100644
--- a/apps/web/src/components/allocations/AllocationModal.tsx
+++ b/apps/web/src/components/allocations/AllocationModal.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useEffect, useRef } from "react";
+import { useState, useEffect, useRef, useMemo } from "react";
 import { useFocusTrap } from "~/hooks/useFocusTrap.js";
 import { useInvalidatePlanningViews } from "~/hooks/useInvalidatePlanningViews.js";
 import { AllocationStatus } from "@planarchy/shared";
@@ -73,6 +73,36 @@ export function AllocationModal({ allocation, onClose, onSuccess }: AllocationMo
     { staleTime: 60_000 },
   );
 
+  // Fetch existing allocations for the selected resource+project to detect overlaps
+  const shouldCheckOverlap = !isDemandEntry && !!resourceId && !!projectId;
+  const { data: existingAllocations } = trpc.allocation.listView.useQuery(
+    { projectId, resourceId },
+    { enabled: shouldCheckOverlap, staleTime: 30_000 },
+  );
+
+  const overlapWarning = useMemo(() => {
+    if (!shouldCheckOverlap || !existingAllocations || !startDate || !endDate) return null;
+    const formStart = new Date(startDate);
+    const formEnd = new Date(endDate);
+    if (isNaN(formStart.getTime()) || isNaN(formEnd.getTime())) return null;
+
+    const allocList = (existingAllocations as { allocations?: Array<{ id: string; resourceId?: string | null; startDate: string | Date; endDate: string | Date }> }).allocations ?? [];
+    for (const existing of allocList) {
+      // Skip the allocation being edited
+      if (isEditing && allocation && existing.id === allocation.id) continue;
+      // Only check assignments for this resource
+      if (existing.resourceId !== resourceId) continue;
+      const exStart = new Date(existing.startDate);
+      const exEnd = new Date(existing.endDate);
+      // Check date overlap
+      if (formStart <= exEnd && formEnd >= exStart) {
+        const fmt = (d: Date) => d.toISOString().slice(0, 10);
+        return `This resource is already assigned to this project from ${fmt(exStart)} to ${fmt(exEnd)}. Consider updating the existing assignment instead.`;
+      }
+    }
+    return null;
+  }, [shouldCheckOverlap, existingAllocations, startDate, endDate, isEditing, allocation, resourceId]);
+
   const invalidatePlanningViews = useInvalidatePlanningViews();
 
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -473,6 +503,13 @@ export function AllocationModal({ allocation, onClose, onSuccess }: AllocationMo
             </div>
           )}
 
+          {/* Overlap warning */}
+          {overlapWarning && (
+            <div className="rounded-lg border border-amber-300 bg-amber-50 px-3 py-2 text-sm text-amber-800 dark:border-amber-700 dark:bg-amber-950/30 dark:text-amber-300">
+              {"\u26A0"} {overlapWarning}
+            </div>
+          )}
+
           {/* Footer */}
           <div className="flex items-center justify-end gap-3 pt-2">
             <button
diff --git a/packages/api/src/router/assistant-tools.ts b/packages/api/src/router/assistant-tools.ts
index 6f0956e..00366ae 100644
--- a/packages/api/src/router/assistant-tools.ts
+++ b/packages/api/src/router/assistant-tools.ts
@@ -4,7 +4,7 @@
  */
 
 import { prisma } from "@planarchy/db";
-import { calculateAllocation, countWorkingDays } from "@planarchy/engine/allocation";
+import { calculateAllocation, checkDuplicateAssignment, countWorkingDays } from "@planarchy/engine/allocation";
 import { computeBudgetStatus } from "@planarchy/engine";
 import type { PermissionKey } from "@planarchy/shared";
 import { parseTaskAction } from "@planarchy/shared";
@@ -2195,6 +2195,16 @@ const executors = {
     const startDate = new Date(params.startDate);
     const endDate = new Date(params.endDate);
 
+    // Check for overlapping duplicate assignments (same resource + project + overlapping dates)
+    const existingAssignments = await ctx.db.assignment.findMany({
+      where: { resourceId: resource.id, status: { not: "CANCELLED" } },
+      select: { id: true, resourceId: true, projectId: true, startDate: true, endDate: true, status: true },
+    });
+    const dupCheck = checkDuplicateAssignment(resource.id, project.id, startDate, endDate, existingAssignments);
+    if (dupCheck.isDuplicate) {
+      return { error: dupCheck.message + " Use update_allocation_status to modify the existing assignment." };
+    }
+
     // Check for existing CANCELLED allocation with same unique key — reactivate it
     const existing = await ctx.db.assignment.findUnique({
       where: {
@@ -3002,6 +3012,16 @@ const executors = {
     }
     if (!resource) return { error: `Resource not found: ${params.resourceId}` };
 
+    // Check for overlapping duplicate assignments (same resource + project + overlapping dates)
+    const existingAssignments = await ctx.db.assignment.findMany({
+      where: { resourceId: resource.id, status: { not: "CANCELLED" } },
+      select: { id: true, resourceId: true, projectId: true, startDate: true, endDate: true, status: true },
+    });
+    const dupCheck = checkDuplicateAssignment(resource.id, demand.project.id, demand.startDate, demand.endDate, existingAssignments);
+    if (dupCheck.isDuplicate) {
+      return { error: dupCheck.message + " Use update_allocation_status to modify the existing assignment." };
+    }
+
     const roleName = demand.roleEntity?.name ?? demand.role ?? null;
     const dailyCostCents = Math.round(resource.lcrCents * demand.hoursPerDay);
     const assignment = await ctx.db.assignment.create({
diff --git a/packages/application/src/use-cases/allocation/create-assignment.ts b/packages/application/src/use-cases/allocation/create-assignment.ts
index 3ffccc4..2892015 100644
--- a/packages/application/src/use-cases/allocation/create-assignment.ts
+++ b/packages/application/src/use-cases/allocation/create-assignment.ts
@@ -1,4 +1,4 @@
-import { calculateAllocation, validateAvailability } from "@planarchy/engine";
+import { calculateAllocation, validateAvailability, checkDuplicateAssignment } from "@planarchy/engine";
 import type { PrismaClient, Prisma } from "@planarchy/db";
 import {
   type Allocation,
@@ -105,6 +105,28 @@ export async function createAssignment(
     },
   );
 
+  const duplicateResult = checkDuplicateAssignment(
+    input.resourceId,
+    input.projectId,
+    input.startDate,
+    input.endDate,
+    existingBookings.map(b => ({
+      id: b.id,
+      resourceId: b.resourceId ?? "",
+      projectId: b.projectId,
+      startDate: b.startDate,
+      endDate: b.endDate,
+      status: b.status,
+    })),
+  );
+
+  if (duplicateResult.isDuplicate) {
+    throw new TRPCError({
+      code: "CONFLICT",
+      message: duplicateResult.message ?? "Resource is already assigned to this project with overlapping dates",
+    });
+  }
+
   const availability = resource.availability as unknown as WeekdayAvailability;
   const availabilityWindows = existingBookings.map((booking) => ({
     startDate: booking.startDate,
diff --git a/packages/application/src/use-cases/allocation/fill-demand-requirement.ts b/packages/application/src/use-cases/allocation/fill-demand-requirement.ts
index 33bdbb3..1c9fd30 100644
--- a/packages/application/src/use-cases/allocation/fill-demand-requirement.ts
+++ b/packages/application/src/use-cases/allocation/fill-demand-requirement.ts
@@ -1,8 +1,10 @@
 import type { PrismaClient } from "@planarchy/db";
 import { AllocationStatus, type FillDemandRequirementInput } from "@planarchy/shared";
+import { checkDuplicateAssignment } from "@planarchy/engine";
 import { TRPCError } from "@trpc/server";
 import { type AssignmentWithRelations } from "./create-assignment.js";
 import { fillDemandRequirementWithLegacySync } from "./fill-demand-requirement-with-legacy-sync.js";
+import { listAssignmentBookings } from "./list-assignment-bookings.js";
 
 export interface FillDemandRequirementResult {
   assignment: AssignmentWithRelations;
@@ -54,5 +56,34 @@ export async function fillDemandRequirement(
     });
   }
 
+  const existingBookings = await listAssignmentBookings(
+    db as unknown as Parameters<typeof listAssignmentBookings>[0],
+    {
+      resourceIds: [input.resourceId],
+    },
+  );
+
+  const duplicateResult = checkDuplicateAssignment(
+    input.resourceId,
+    demandRequirement.projectId,
+    demandRequirement.startDate,
+    demandRequirement.endDate,
+    existingBookings.map(b => ({
+      id: b.id,
+      resourceId: b.resourceId ?? "",
+      projectId: b.projectId,
+      startDate: b.startDate,
+      endDate: b.endDate,
+      status: b.status,
+    })),
+  );
+
+  if (duplicateResult.isDuplicate) {
+    throw new TRPCError({
+      code: "CONFLICT",
+      message: duplicateResult.message ?? "Resource is already assigned to this project with overlapping dates",
+    });
+  }
+
   return fillDemandRequirementWithLegacySync(db, demandRequirement, input);
 }
diff --git a/packages/engine/src/__tests__/duplicate-check.test.ts b/packages/engine/src/__tests__/duplicate-check.test.ts
new file mode 100644
index 0000000..3e0bf62
--- /dev/null
+++ b/packages/engine/src/__tests__/duplicate-check.test.ts
@@ -0,0 +1,99 @@
+import { describe, it, expect } from "vitest";
+import { checkDuplicateAssignment, type ExistingAssignment } from "../allocation/duplicate-check.js";
+
+const base: ExistingAssignment = {
+  id: "a1",
+  resourceId: "r1",
+  projectId: "p1",
+  startDate: new Date("2026-03-01"),
+  endDate: new Date("2026-06-30"),
+  status: "CONFIRMED",
+};
+
+describe("checkDuplicateAssignment", () => {
+  it("returns no duplicate when no existing assignments", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", []);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate for different project", () => {
+    const result = checkDuplicateAssignment("r1", "p2", "2026-04-01", "2026-05-01", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate for different resource", () => {
+    const result = checkDuplicateAssignment("r2", "p1", "2026-04-01", "2026-05-01", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate when dates don't overlap (before)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-01-01", "2026-02-28", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("returns no duplicate when dates don't overlap (after)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-07-01", "2026-08-01", [base]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("detects duplicate with full overlap", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-03-15", "2026-05-15", [base]);
+    expect(result.isDuplicate).toBe(true);
+    expect(result.conflictingAssignment?.id).toBe("a1");
+    expect(result.message).toContain("already assigned");
+  });
+
+  it("detects duplicate with partial overlap (start before, end during)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-02-15", "2026-04-01", [base]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("detects duplicate with partial overlap (start during, end after)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-05-01", "2026-07-15", [base]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("detects duplicate on exact same dates", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-03-01", "2026-06-30", [base]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("ignores CANCELLED assignments", () => {
+    const cancelled = { ...base, status: "CANCELLED" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [cancelled]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("ignores DRAFT assignments", () => {
+    const draft = { ...base, status: "DRAFT" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [draft]);
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("excludes self by ID (for updates)", () => {
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [base], "a1");
+    expect(result.isDuplicate).toBe(false);
+  });
+
+  it("does not exclude other assignments when excludeId is set", () => {
+    const other: ExistingAssignment = { ...base, id: "a2" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [other], "a1");
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("works with string dates", () => {
+    const strAssignment: ExistingAssignment = {
+      ...base,
+      startDate: "2026-03-01",
+      endDate: "2026-06-30",
+    };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [strAssignment]);
+    expect(result.isDuplicate).toBe(true);
+  });
+
+  it("checks PROPOSED status as active", () => {
+    const proposed = { ...base, status: "PROPOSED" };
+    const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [proposed]);
+    expect(result.isDuplicate).toBe(true);
+  });
+});
diff --git a/packages/engine/src/allocation/duplicate-check.ts b/packages/engine/src/allocation/duplicate-check.ts
new file mode 100644
index 0000000..68f6c9e
--- /dev/null
+++ b/packages/engine/src/allocation/duplicate-check.ts
@@ -0,0 +1,78 @@
+/**
+ * Checks whether a resource is already assigned to the same project
+ * with overlapping dates. Used to prevent duplicate bookings.
+ */
+
+export interface ExistingAssignment {
+  id: string;
+  resourceId: string;
+  projectId: string;
+  startDate: Date | string;
+  endDate: Date | string;
+  status: string;
+}
+
+export interface DuplicateCheckResult {
+  isDuplicate: boolean;
+  conflictingAssignment?: ExistingAssignment;
+  message?: string;
+}
+
+const ACTIVE_STATUSES = new Set(["CONFIRMED", "ACTIVE", "PROPOSED"]);
+
+function toTime(d: Date | string): number {
+  const dt = typeof d === "string" ? new Date(d) : d;
+  dt.setHours(0, 0, 0, 0);
+  return dt.getTime();
+}
+
+/**
+ * Check if assigning a resource to a project with the given dates
+ * would create a duplicate (same resource + same project + overlapping dates).
+ *
+ * @param resourceId - The resource being assigned
+ * @param projectId - The project being assigned to
+ * @param startDate - Start of the new assignment
+ * @param endDate - End of the new assignment
+ * @param existingAssignments - All current assignments for this resource
+ * @param excludeAssignmentId - Exclude this ID (for updates — don't conflict with self)
+ */
+export function checkDuplicateAssignment(
+  resourceId: string,
+  projectId: string,
+  startDate: Date | string,
+  endDate: Date | string,
+  existingAssignments: ExistingAssignment[],
+  excludeAssignmentId?: string,
+): DuplicateCheckResult {
+  const newStart = toTime(startDate);
+  const newEnd = toTime(endDate);
+
+  for (const existing of existingAssignments) {
+    // Skip self (for updates)
+    if (excludeAssignmentId && existing.id === excludeAssignmentId) continue;
+
+    // Skip different resource or project
+    if (existing.resourceId !== resourceId) continue;
+    if (existing.projectId !== projectId) continue;
+
+    // Skip cancelled/inactive
+    if (!ACTIVE_STATUSES.has(existing.status)) continue;
+
+    // Check date overlap: existingStart <= newEnd && existingEnd >= newStart
+    const existStart = toTime(existing.startDate);
+    const existEnd = toTime(existing.endDate);
+
+    if (existStart <= newEnd && existEnd >= newStart) {
+      const startStr = new Date(existing.startDate).toISOString().slice(0, 10);
+      const endStr = new Date(existing.endDate).toISOString().slice(0, 10);
+      return {
+        isDuplicate: true,
+        conflictingAssignment: existing,
+        message: `Resource is already assigned to this project from ${startStr} to ${endStr}. Consider updating the existing assignment instead.`,
+      };
+    }
+  }
+
+  return { isDuplicate: false };
+}
diff --git a/packages/engine/src/allocation/index.ts b/packages/engine/src/allocation/index.ts
index 408b102..efb44dc 100644
--- a/packages/engine/src/allocation/index.ts
+++ b/packages/engine/src/allocation/index.ts
@@ -2,3 +2,4 @@ export * from "./calculator.js";
 export * from "./availability-validator.js";
 export * from "./recurrence.js";
 export * from "./chargeability.js";
+export * from "./duplicate-check.js";
diff --git a/plan.md b/plan.md
index 53be991..bd545e0 100644
--- a/plan.md
+++ b/plan.md
@@ -1,78 +1,17 @@
-# Activity History System — Detailed Plan
+# Duplicate Assignment Prevention — Plan
 
 ## Anforderungsanalyse
 
-**Ziel:** Ein lueckenloses Aenderungsprotokoll, das jede Mutation im System erfasst und ueber UI und AI Assistant abfragbar macht. Nutzer sollen fragen koennen: "Wer hat die Buchung von Person X geaendert?" oder "Was ist in den letzten Tagen bei Projekt Y passiert?"
+**Problem:** Ressourcen koennen demselben Projekt mehrfach zugewiesen werden mit ueberlappenden Zeitraeumen. Beispiel: Wong Wong ist zweimal fuer "Porsche Taycan Sport Film" am 15. April eingetragen.
 
-**Ist-Zustand:**
-- AuditLog-Modell existiert (entityType, entityId, action, userId, changes JSONB, createdAt)
-- 10 von 36 Routern loggen Aenderungen (44% Abdeckung)
-- userId wird nur in ~60% der Faelle erfasst
-- Kein Query-Endpoint (write-only)
-- Keine UI zum Anzeigen der Historie
-- AI Assistant kann keine Historie abfragen
-- Inkonsistente before/after Snapshots
+**Ursache:** Weder die Application-Layer-Funktionen (`createAssignment`, `fillDemandRequirement`) noch die API-Router pruefen, ob dieselbe Resource bereits eine aktive Zuweisung zum selben Projekt im selben Zeitraum hat. Die bestehende `validateAvailability` prueft nur die Gesamt-Stunden (Overbooking), nicht Projekt-Duplikate.
 
-**Soll-Zustand:**
-- 100% Mutation-Abdeckung ueber alle Router
-- Konsistente before/after Snapshots mit User-Attribution
-- Query-API mit Filtern (entityType, entityId, userId, dateRange, action)
-- Admin-UI: `/admin/activity-log` mit suchbarer, filterbarer Timeline
-- Entity-Detail-Seiten: "History"-Tab/-Drawer auf Project/Resource/Allocation
-- AI Assistant Tool: `query_change_history` fuer natuerlichsprachliche Abfragen
-- Change-Source Tracking: UI vs API vs AI vs Import
+**Loesung:** Duplicate-Check an **3 Stellen** einfuegen (defense-in-depth):
+1. **Application Layer** — `checkDuplicateAssignment()` Funktion im Engine-Paket
+2. **API Layer** — Validierung in den Mutations vor dem Create
+3. **AI Assistant** — `create_allocation` und `fill_demand` Tools pruefen vor Ausfuehrung
 
----
-
-## Architektur-Entscheidungen
-
-### 1. Audit Middleware statt manuelle Calls
-**Entscheidung:** tRPC Middleware die automatisch vor/nach jeder Mutation auditiert
-**Grund:** Eliminiert vergessene `auditLog.create()` Calls, garantiert 100% Abdeckung
-**Umsetzung:** Middleware auf `protectedProcedure` die:
-- Vor der Mutation: Entity-Snapshot speichert (before)
-- Nach der Mutation: Neuen Snapshot speichert (after)
-- Diff berechnet und AuditLog-Entry erstellt
-
-### 2. Standardisiertes Changes-Format
-```typescript
-interface AuditChanges {
-  before?: Record<string, unknown>;  // Snapshot vor der Aenderung
-  after?: Record<string, unknown>;   // Snapshot nach der Aenderung
-  diff?: Record<string, { old: unknown; new: unknown }>;  // Nur geaenderte Felder
-  metadata?: {
-    source: "ui" | "api" | "ai" | "import" | "cron";  // Wer hat die Aenderung ausgeloest
-    reason?: string;   // Optionaler Kommentar
-    ip?: string;       // Request IP (optional)
-    batchId?: string;  // Fuer Bulk-Operationen
-  };
-}
-```
-
-### 3. Schema-Erweiterungen
-```prisma
-model AuditLog {
-  // Existierende Felder behalten
-  id         String      @id @default(cuid())
-  entityType String
-  entityId   String
-  action     AuditAction
-  userId     String?
-  user       User?       @relation(fields: [userId], references: [id])
-  changes    Json        @db.JsonB
-  createdAt  DateTime    @default(now())
-
-  // NEU: Zusaetzliche Felder
-  source     String?     // "ui" | "api" | "ai" | "import" | "cron"
-  entityName String?     // Menschenlesbarer Name (z.B. "Porsche Taycan Project")
-  summary    String?     // Einzeiler: "Changed status from DRAFT to ACTIVE"
-
-  @@index([entityType, entityId])
-  @@index([userId])
-  @@index([createdAt])
-  @@index([entityType, createdAt])  // NEU: Fuer sortierte Timeline-Queries
-}
-```
+**Scope:** Betrifft `packages/engine`, `packages/application`, `packages/api`, UI (Warnmeldung).
 
 ---
 
@@ -80,158 +19,150 @@ model AuditLog {
 
 | Paket | Dateien | Art der Aenderung |
 |-------|---------|------------------|
-| `packages/db` | `prisma/schema.prisma` | **edit** — AuditLog um source, entityName, summary erweitern |
-| `packages/api` | `src/lib/audit.ts` | **create** — `createAuditEntry()` Helper + `auditMiddleware` |
-| `packages/api` | `src/router/audit-log.ts` | **create** — Query-Router (list, getByEntity, getTimeline) |
-| `packages/api` | `src/router/index.ts` | **edit** — auditLog Router registrieren |
-| `packages/api` | `src/router/assistant-tools.ts` | **edit** — `query_change_history` Tool hinzufuegen |
-| `packages/api` | 26 Router-Dateien | **edit** — fehlende audit Calls nachruesten |
-| `apps/web` | `src/app/(app)/admin/activity-log/page.tsx` | **create** — Activity Log Seite |
-| `apps/web` | `src/components/admin/ActivityLogClient.tsx` | **create** — Suchbare Timeline |
-| `apps/web` | `src/components/ui/EntityHistory.tsx` | **create** — Wiederverwendbare History-Komponente |
-| `apps/web` | `src/components/layout/AppShell.tsx` | **edit** — Nav-Link fuer Activity Log |
+| `packages/engine` | `src/allocation/duplicate-check.ts` | **create** — Pure Funktion `checkDuplicateAssignment()` |
+| `packages/engine` | `src/index.ts` | **edit** — Export hinzufuegen |
+| `packages/application` | `src/use-cases/allocation/create-assignment.ts` | **edit** — Duplicate-Check vor DB-Write |
+| `packages/application` | `src/use-cases/allocation/fill-demand-requirement.ts` | **edit** — Duplicate-Check vor DB-Write |
+| `packages/api` | `src/router/allocation.ts` | **edit** — Duplicate-Check in `create`, `createAssignment` Mutations |
+| `packages/api` | `src/router/assistant-tools.ts` | **edit** — Check in `create_allocation`, `fill_demand` Tools |
+| `packages/api` | `src/router/timeline.ts` | **edit** — Check in `batchShiftAllocations` (falls Shift Duplikat erzeugt) |
+| `apps/web` | `src/components/allocations/AllocationModal.tsx` | **edit** — Warning anzeigen wenn Duplikat erkannt |
+| `apps/web` | `src/components/staffing/StaffingPanel.tsx` | **edit** — Warning im Assign-Formular |
+| `packages/engine` | `src/__tests__/duplicate-check.test.ts` | **create** — Unit Tests |
 
 ---
 
-## Task-Liste (atomare Schritte)
+## Task-Liste
 
-### Phase 1: Infrastruktur (Basis)
+### Phase 1: Engine — Pure Duplicate-Check Funktion
 
-- [ ] **Task 1:** Schema erweitern → `packages/db/prisma/schema.prisma`
-  - `source String?`, `entityName String?`, `summary String?` hinzufuegen
-  - Index `@@index([entityType, createdAt])` hinzufuegen
-  - `prisma db push` + `prisma generate`
+- [ ] **Task 1:** Duplicate-Check Funktion erstellen → `packages/engine/src/allocation/duplicate-check.ts`
+  ```typescript
+  interface ExistingAssignment {
+    id: string;
+    resourceId: string;
+    projectId: string;
+    startDate: Date;
+    endDate: Date;
+    status: string; // nur CONFIRMED, ACTIVE, PROPOSED zaehlen
+  }
 
-- [ ] **Task 2:** Audit Helper erstellen → `packages/api/src/lib/audit.ts`
-  - `createAuditEntry(db, params)` — standardisierter Audit-Entry-Creator
-  - Params: `{ entityType, entityId, entityName, action, userId, before?, after?, source?, summary? }`
-  - Automatische Diff-Berechnung wenn before + after vorhanden
-  - Automatische Summary-Generierung aus Diff (z.B. "Updated name, status, budgetCents")
-  - `computeDiff(before, after)` — gibt nur geaenderte Felder zurueck
+  interface DuplicateCheckResult {
+    isDuplicate: boolean;
+    conflictingAssignment?: ExistingAssignment;
+    message?: string; // z.B. "Resource Wong Wong is already assigned to Porsche Taycan (2026-03-01 to 2026-06-30)"
+  }
 
-- [ ] **Task 3:** Query Router erstellen → `packages/api/src/router/audit-log.ts`
-  - `list` query (controllerProcedure): paginiert, filterbar nach entityType, entityId, userId, action, dateRange, source
-  - `getByEntity` query: alle Entries fuer eine Entity, chronologisch
-  - `getTimeline` query: globale Timeline aller Aenderungen, gruppierbar nach Tag
-  - `getActivitySummary` query: Zusammenfassung (counts pro entityType, pro action, pro User) fuer einen Zeitraum
-  - Registrieren in `router/index.ts`
+  export function checkDuplicateAssignment(
+    resourceId: string,
+    projectId: string,
+    startDate: Date,
+    endDate: Date,
+    existingAssignments: ExistingAssignment[],
+    excludeAssignmentId?: string, // fuer Updates: eigene ID ausschliessen
+  ): DuplicateCheckResult
+  ```
+  - Prueft: Gibt es in `existingAssignments` eine Zuweisung mit **gleicher resourceId + gleicher projectId** deren Zeitraum sich mit [startDate, endDate] ueberschneidet?
+  - Ignoriert: CANCELLED Status, eigene ID (bei Updates)
+  - Overlap-Logik: `existingStart <= newEnd && existingEnd >= newStart`
 
-### Phase 2: Audit-Abdeckung erweitern
+- [ ] **Task 2:** Unit Tests → `packages/engine/src/__tests__/duplicate-check.test.ts`
+  - Kein Duplikat: verschiedene Projekte
+  - Kein Duplikat: gleicher Projekt, aber nicht ueberlappend (vor/nach)
+  - Duplikat: gleicher Projekt, vollstaendig ueberlappend
+  - Duplikat: gleicher Projekt, teilweise ueberlappend
+  - Kein Duplikat: gleicher Projekt, aber CANCELLED
+  - Kein Duplikat: Update der eigenen Zuweisung (excludeAssignmentId)
 
-- [ ] **Task 4:** Kritische Router nachruesteen (Parallel-fähig, 4 Agents)
-  - **Agent A:** `vacation.ts` (8 Mutations), `entitlement.ts` (2), `user.ts` (9)
-  - **Agent B:** `client.ts` (5), `org-unit.ts` (3), `country.ts` (5), `management-level.ts` (5)
-  - **Agent C:** `rate-card.ts` (7), `blueprint.ts` (6), `settings.ts` (3), `calculation-rules.ts` (3)
-  - **Agent D:** `webhook.ts` (4), `comment.ts` (3), `notification.ts` (nur create/task), `dispo.ts` (4)
-  - Jeder Agent: `import { createAuditEntry } from "../lib/audit.js"` verwenden
-  - userId immer aus `ctx.dbUser?.id` nehmen
+- [ ] **Task 3:** Export → `packages/engine/src/index.ts`
 
-- [ ] **Task 5:** Bestehende Audit-Calls standardisieren
-  - Alle 37 existierenden `auditLog.create` Calls auf `createAuditEntry()` Helper umstellen
-  - userId konsistent aus Context nehmen
-  - before/after Snapshots wo fehlend ergaenzen
-  - `source: "ui"` als Default setzen
+### Phase 2: Application Layer — Integration in Create-Flows
 
-### Phase 3: UI
+- [ ] **Task 4:** `createAssignment` erweitern → `packages/application/src/use-cases/allocation/create-assignment.ts`
+  - Nach dem Laden von `existingBookings` (Zeile 101-106): `checkDuplicateAssignment()` aufrufen
+  - Bei `isDuplicate: true`: `throw new TRPCError({ code: "CONFLICT", message: result.message })`
+  - Bestehende Bookings bereits vorhanden — nur filtern auf gleichen `projectId`
 
-- [ ] **Task 6:** Activity Log Admin-Seite → `ActivityLogClient.tsx`
-  - Globale, suchbare Timeline aller Aenderungen
-  - Filter: Entity-Typ (Project/Resource/Allocation/...), User, Action, Datum
-  - Jeder Eintrag zeigt: Zeitstempel, User (Avatar + Name), Entity (verlinkt), Action-Badge, Summary
-  - Expandierbares Detail: before/after Diff-View (JSON oder tabellarisch)
-  - Pagination (50 pro Seite)
-  - Sidebar Nav-Link unter Admin: "Activity Log"
+- [ ] **Task 5:** `fillDemandRequirement` erweitern → `packages/application/src/use-cases/allocation/fill-demand-requirement.ts`
+  - Vor dem Assignment-Create: gleicher Check
+  - DemandRequirement hat bereits `projectId` — diesen nutzen
 
-- [ ] **Task 7:** Entity History Komponente → `EntityHistory.tsx`
-  - Wiederverwendbar fuer Project/Resource/Allocation Detail-Seiten
-  - Props: `entityType: string, entityId: string`
-  - Chronologische Liste der Aenderungen fuer diese Entity
-  - Kompakte Darstellung: User, Action, Summary, Zeitstempel
-  - Optional: als Tab oder Drawer auf Detail-Seiten einbinden
+### Phase 3: API + AI Assistant
 
-- [ ] **Task 8:** History-Tab auf Detail-Seiten integrieren
-  - `/projects/[id]` → "History" Tab mit `<EntityHistory entityType="project" entityId={id} />`
-  - `/resources/[id]` → "History" Tab
-  - Optional spaeter: Allocation Detail, Estimate Detail
+- [ ] **Task 6:** AI Assistant Tools erweitern → `packages/api/src/router/assistant-tools.ts`
+  - `create_allocation` Tool: Vor `createAssignment` Call, bestehende Assignments pruefen
+  - `fill_demand` Tool: Gleicher Check
+  - Bei Duplikat: Tool gibt klare Fehlermeldung zurueck statt Exception:
+    `"Cannot assign: Wong Wong is already assigned to Porsche Taycan Sport Film from 2026-01-15 to 2026-06-30. Use update_allocation_status to modify the existing assignment instead."`
 
-### Phase 4: AI Assistant Integration
+### Phase 4: UI Warnungen
 
-- [ ] **Task 9:** AI Tool erstellen → `assistant-tools.ts`
-  - `query_change_history` Tool:
-    - Input: `{ entityType?, entityId?, userId?, search?, daysBack?, limit? }`
-    - Ruft `auditLog.list` mit Filtern auf
-    - Formatiert Ergebnis menschenlesbar:
-      ```
-      [2026-03-22 14:30] admin@planarchy.dev UPDATED Project "Porsche Taycan"
-        → Changed status from DRAFT to ACTIVE
-        → Changed budgetCents from 500000 to 750000
-      ```
-  - `get_entity_timeline` Tool:
-    - Input: `{ entityType, entityId, limit? }`
-    - Gibt chronologische History fuer eine Entity zurueck
-  - Beide Tools mit Permission `VIEW_PROJECTS` oder `VIEW_RESOURCES` je nach entityType
+- [ ] **Task 7:** AllocationModal Warning → `apps/web/src/components/allocations/AllocationModal.tsx`
+  - Wenn User Resource + Project + Dates auswaehlt: pruefen ob Duplikat existiert
+  - Query: `trpc.allocation.listView({ projectId })` — bereits geladen
+  - Gelbe Warning-Box: "This resource is already assigned to this project from X to Y"
+  - Submit-Button nicht blockieren (Warning, nicht Error) — User kann bewusst doppelt buchen
+
+- [ ] **Task 8:** StaffingPanel Assign Warning → `apps/web/src/components/staffing/StaffingPanel.tsx`
+  - Im AssignForm: nach Project-Auswahl pruefen ob Resource bereits dort zugewiesen
+  - Gleiche Warning-Box wie AllocationModal
+
+### Phase 5: Bereinigung bestehender Duplikate
+
+- [ ] **Task 9:** Cleanup-Script → `packages/db/scripts/deduplicate-assignments.ts`
+  - Findet alle Duplikate: gleiche resourceId + projectId mit ueberlappenden Dates
+  - Merged sie: behaelt die aeltere Zuweisung, entfernt die neuere (oder merged Zeitraeume)
+  - Dry-run Modus: zeigt was geaendert wuerde ohne zu aendern
+  - Kann via `pnpm --filter @planarchy/db exec tsx scripts/deduplicate-assignments.ts` ausgefuehrt werden
 
 ---
 
 ## Abhaengigkeiten
 
 ```
-Task 1 (Schema) ──► Task 2 (Helper) ──► Task 3 (Query Router)
-                                    └──► Task 4a-d (Parallel: 26 Router)
-                                    └──► Task 5 (Bestehende Calls)
-                    Task 3 ──► Task 6 (UI: Activity Log)
-                           ──► Task 7 (UI: Entity History)
-                           ──► Task 9 (AI Tools)
-                    Task 7 ──► Task 8 (Integration in Detail-Seiten)
+Task 1 (Engine Funktion) → Task 2 (Tests) → Task 3 (Export)
+                        ↘
+Task 3 → Task 4 + Task 5 (parallel, Application Layer)
+Task 3 → Task 6 (AI Assistant)
+Task 3 → Task 7 + Task 8 (parallel, UI Warnungen)
+Task 9 (Cleanup) ist unabhaengig, kann jederzeit ausgefuehrt werden
 ```
 
-- Tasks 4a-d koennen **parallel** ausgefuehrt werden (unterschiedliche Dateien)
-- Tasks 6, 7, 9 koennen **parallel** nach Task 3
-- Task 8 benoetigt Task 7
+- Tasks 4+5 koennen **parallel** (verschiedene Dateien)
+- Tasks 6, 7, 8 koennen **parallel** (verschiedene Dateien)
+- Task 9 sollte **nach** den anderen Tasks laufen (damit neue Duplikate verhindert werden)
 
 ---
 
 ## Akzeptanzkriterien
 
+- [ ] `pnpm test:unit` laeuft gruen (inkl. neue duplicate-check Tests)
 - [ ] `pnpm --filter @planarchy/web exec tsc --noEmit` — keine neuen Errors
-- [ ] `pnpm test:unit` — alle Tests gruen
-- [ ] **100% Mutation-Abdeckung:** Jede Mutation in jedem Router erzeugt einen AuditLog-Entry
-- [ ] **Konsistente userId:** Jeder Entry hat den ausfuehrenden User
-- [ ] **before/after:** UPDATE-Actions haben immer before + after Snapshots
-- [ ] **Query-API:** `trpc.auditLog.list` liefert paginierte, filterbare Ergebnisse
-- [ ] **Admin UI:** `/admin/activity-log` zeigt globale Timeline mit Filtern
-- [ ] **Entity History:** Project/Resource Detail-Seiten zeigen Aenderungs-Historie
-- [ ] **AI Assistant:** "Wer hat die Buchung von Person X geaendert?" wird korrekt beantwortet
-- [ ] **AI Assistant:** "Was ist bei Projekt Y in den letzten Tagen passiert?" liefert Ergebnis
+- [ ] **API:** `createAssignment` wirft CONFLICT wenn Resource bereits zum gleichen Projekt zugewiesen
+- [ ] **API:** `fillDemandRequirement` wirft CONFLICT bei Duplikat
+- [ ] **AI Assistant:** `create_allocation` gibt klare Fehlermeldung bei Duplikat
+- [ ] **AI Assistant:** `fill_demand` gibt klare Fehlermeldung bei Duplikat
+- [ ] **UI:** AllocationModal zeigt gelbe Warning bei erkanntem Duplikat
+- [ ] **UI:** StaffingPanel AssignForm zeigt Warning bei Duplikat
+- [ ] **Cleanup:** Bestehende Duplikate in der DB bereinigt
+- [ ] **Timeline:** Wong Wong hat keine doppelten Strips mehr am 15. April
 
 ---
 
 ## Risiken & offene Fragen
 
 ### Risiken
-- **Performance:** Audit-Middleware auf jeder Mutation koennte Latenz erhoehen
-  → Mitigation: Audit-Writes fire-and-forget (non-blocking), oder nach Response
-- **Storage:** JSONB Snapshots koennen gross werden
-  → Mitigation: Nur geaenderte Felder in `diff` speichern, nicht volle Snapshots
-- **Migration:** 37 bestehende Calls umstellen birgt Regressions-Risiko
-  → Mitigation: Schrittweise, mit Tests pro Router
+- **False Positives:** Legitime Doppelbuchungen (z.B. verschiedene Rollen auf demselben Projekt) werden blockiert
+  → Mitigation: Warning im UI, Error nur im API — User kann override-en, AI Assistant gibt Hinweis
+- **Race Condition:** Zwei gleichzeitige Requests koennten beide den Check passieren
+  → Mitigation: DB-Level unique constraint ist nicht moeglich (flexible Zeitraeume), aber Transaction-Isolation schuetzt
 
 ### Offene Fragen
-1. **Retention:** Wie lange sollen Audit-Logs aufbewahrt werden? (Vorschlag: 2 Jahre)
-2. **Granularitaet:** Sollen READ-Zugriffe geloggt werden? (Vorschlag: Nein, nur Mutations)
-3. **DSGVO:** Muessen Audit-Logs bei User-Loeschung anonymisiert werden?
-4. **Notifications:** Sollen bestimmte Aenderungen (z.B. Projekt-Status) automatisch Notifications ausloesen?
-5. **Middleware vs Manual:** Soll der Audit-Helper manuell oder als tRPC-Middleware eingebaut werden?
-   → Empfehlung: Manuell mit Helper-Funktion, da Middleware die Entity-Snapshots nicht automatisch kennt
-
----
-
-## Geschaetzter Aufwand
-
-| Phase | Aufwand | Parallelisierbar |
-|-------|---------|-----------------|
-| Phase 1: Infrastruktur | 1 Tag | Nein (sequenziell) |
-| Phase 2: Audit-Abdeckung | 1 Tag | Ja (4 Agents parallel) |
-| Phase 3: UI | 1 Tag | Ja (2 Agents parallel) |
-| Phase 4: AI Integration | 0.5 Tag | Ja (mit Phase 3) |
-| **Gesamt** | **~3.5 Tage** | |
+1. **Soll der Check nur warnen oder blockieren?**
+   → Empfehlung: API blockiert (CONFLICT), UI warnt (gelbe Box, Submit moeglich), AI blockiert
+2. **Was passiert bei Updates/Shifts?**
+   → excludeAssignmentId nutzen um die eigene Zuweisung auszuschliessen
+3. **Welche Status zaehlen als "aktiv"?**
+   → CONFIRMED, ACTIVE, PROPOSED — nicht CANCELLED, DRAFT
+4. **Sollen verschiedene Rollen erlaubt sein?**
+   → Vorschlag: Ja, aber mit Warning. Gleiche Rolle + gleiches Projekt = Block, verschiedene Rolle = Warning only