diff --git a/src/app/op-log/persistence/operation-log-store.service.spec.ts b/src/app/op-log/persistence/operation-log-store.service.spec.ts
index 8820235de3..586cc05bdc 100644
--- a/src/app/op-log/persistence/operation-log-store.service.spec.ts
+++ b/src/app/op-log/persistence/operation-log-store.service.spec.ts
@@ -724,6 +724,23 @@ describe('OperationLogStoreService', () => {
 
       expect(seqs).toEqual([]);
     });
+
+    it('should throw on duplicate operation IDs', async () => {
+      const ops = [createTestOperation(), createTestOperation()];
+
+      // Insert first time - should succeed
+      const seqs1 = await service.appendBatch(ops, 'remote');
+      expect(seqs1.length).toBe(2);
+
+      // Insert same ops again - should throw
+      await expectAsync(service.appendBatch(ops, 'remote')).toBeRejectedWithError(
+        /Duplicate operation detected/,
+      );
+
+      // Original ops should still be in store
+      const allOps = await service.getOpsAfterSeq(0);
+      expect(allOps.length).toBe(2);
+    });
   });
 
   describe('getOpById', () => {
diff --git a/src/app/op-log/persistence/operation-log-store.service.ts b/src/app/op-log/persistence/operation-log-store.service.ts
index 8589a2b769..8299b4adf5 100644
--- a/src/app/op-log/persistence/operation-log-store.service.ts
+++ b/src/app/op-log/persistence/operation-log-store.service.ts
@@ -24,6 +24,7 @@ import {
   OPS_INDEXES,
 } from './db-keys.const';
 import { runDbUpgrade } from './db-upgrade';
+import { Log } from '../../core/log';
 
 /**
  * Vector clock entry stored in the vector_clock object store.
@@ -274,6 +275,12 @@ export class OperationLogStoreService {
       if (e instanceof DOMException && e.name === 'QuotaExceededError') {
         throw new StorageQuotaExceededError();
       }
+      // ConstraintError: duplicate operation ID - see issue #6213
+      if (e instanceof DOMException && e.name === 'ConstraintError') {
+        throw new Error(
+          '[OpLogStore] Duplicate operation detected (likely race condition). See #6213.',
+        );
+      }
       throw e;
     }
   }
@@ -320,7 +327,7 @@ export class OperationLogStoreService {
     } catch (e) {
       // Fallback for databases created before version 3 index migration
       // This handles the case where the bySourceAndStatus index doesn't exist
-      console.warn(
+      Log.warn(
         'OperationLogStoreService: bySourceAndStatus index not found, using fallback scan',
       );
       const allOps = await this.db.getAll(STORE_NAMES.OPS);
@@ -707,7 +714,7 @@ export class OperationLogStoreService {
       );
     } catch (e) {
       // Fallback for databases created before version 3 index migration
-      console.warn(
+      Log.warn(
         'OperationLogStoreService: bySourceAndStatus index not found, using fallback scan',
       );
       const allOps = await this.db.getAll(STORE_NAMES.OPS);