diff --git a/CHANGELOG.md b/CHANGELOG.md
index 27ad980..39e9a03 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [Backend 0.6.5] - 2026-08-19
+
+### Fixed
+
+- Duplicate operation UUIDs in one import file now return a validation error instead of a database failure.
+
 ## [Frontend 0.9.2 / Backend 0.6.4 / Shared 0.2.2] - 2026-08-19
 
 ### Added
diff --git a/backend/package.json b/backend/package.json
index 327826d..3146015 100644
--- a/backend/package.json
+++ b/backend/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@family-budget/backend",
-  "version": "0.6.4",
+  "version": "0.6.5",
   "private": true,
   "scripts": {
     "dev": "tsx watch src/app.ts",
diff --git a/backend/src/services/import.ts b/backend/src/services/import.ts
index 37deedb..5169804 100644
--- a/backend/src/services/import.ts
+++ b/backend/src/services/import.ts
@@ -133,12 +133,20 @@ function validateSemantics(data: StatementFile): ValidationError | null {
   }
 
   const fps = new Set<string>();
+  const operationIds = new Set<string>();
   for (let i = 0; i < data.transactions.length; i++) {
     const fp = computeFingerprint(data.statement.accountNumber, data.transactions[i]);
     if (fps.has(fp)) {
       return { status: 422, error: 'VALIDATION_ERROR', message: `Duplicate fingerprint found within file at transaction index ${i}` };
     }
     fps.add(fp);
+    const operationId = data.transactions[i].operationId;
+    if (operationId) {
+      if (operationIds.has(operationId.toLowerCase())) {
+        return { status: 422, error: 'VALIDATION_ERROR', message: `Duplicate operationId found within file at transaction index ${i}` };
+      }
+      operationIds.add(operationId.toLowerCase());
+    }
   }
 
   return null;