diff --git a/artifacts/api-server/src/routes/timesheets.ts b/artifacts/api-server/src/routes/timesheets.ts
index 2275159..66a0d79 100644
--- a/artifacts/api-server/src/routes/timesheets.ts
+++ b/artifacts/api-server/src/routes/timesheets.ts
@@ -54,6 +54,20 @@ router.post("/timesheets", async (req, res): Promise<void> => {
     return;
   }
 
+  const [existing] = await db
+    .select()
+    .from(timesheetsTable)
+    .where(and(
+      eq(timesheetsTable.year, parsed.data.year),
+      eq(timesheetsTable.month, parsed.data.month),
+      eq(timesheetsTable.collaborator, parsed.data.collaborator)
+    ));
+
+  if (existing) {
+    res.status(409).json({ error: "Un CRA existe déjà pour ce mois et ce collaborateur" });
+    return;
+  }
+
   const [timesheet] = await db.insert(timesheetsTable).values(parsed.data).returning();
   res.status(201).json(GetTimesheetResponse.parse({ ...timesheet, lines: [] }));
 });
diff --git a/artifacts/cra-app/src/pages/timesheets.tsx b/artifacts/cra-app/src/pages/timesheets.tsx
index 99dc515..aae4e76 100644
--- a/artifacts/cra-app/src/pages/timesheets.tsx
+++ b/artifacts/cra-app/src/pages/timesheets.tsx
@@ -173,7 +173,7 @@ function CreateTimesheetDialog({ open, onOpenChange }: { open: boolean; onOpenCh
       data: {
         year: parseInt(year),
         month: parseInt(month),
-        collaborator: "Jean Dupont" // In a real app, from auth context
+        collaborator: "PHAM Sylvain"
       }
     }, {
       onSuccess: (data) => {
diff --git a/attached_assets/image_1776154810677.png b/attached_assets/image_1776154810677.png
new file mode 100644
index 0000000..7028768
Binary files /dev/null and b/attached_assets/image_1776154810677.png differ
diff --git a/lib/db/src/schema/timesheets.ts b/lib/db/src/schema/timesheets.ts
index 175e82e..54528a7 100644
--- a/lib/db/src/schema/timesheets.ts
+++ b/lib/db/src/schema/timesheets.ts
@@ -1,4 +1,4 @@
-import { pgTable, text, serial, timestamp, integer, real } from "drizzle-orm/pg-core";
+import { pgTable, text, serial, timestamp, integer, real, unique } from "drizzle-orm/pg-core";
 import { createInsertSchema } from "drizzle-zod";
 import { z } from "zod/v4";
 
@@ -11,7 +11,9 @@ export const timesheetsTable = pgTable("timesheets", {
   totalHours: real("total_hours").notNull().default(0),
   createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
   updatedAt: timestamp("updated_at", { withTimezone: true }).notNull().defaultNow().$onUpdate(() => new Date()),
-});
+}, (table) => [
+  unique("timesheets_year_month_collaborator_unique").on(table.year, table.month, table.collaborator),
+]);
 
 export const insertTimesheetSchema = createInsertSchema(timesheetsTable).omit({ id: true, totalHours: true, createdAt: true, updatedAt: true });
 export type InsertTimesheet = z.infer<typeof insertTimesheetSchema>;