diff --git a/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/__tests__/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.spec.ts b/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/__tests__/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.spec.ts
new file mode 100644
index 0000000000..e0c728770e
--- /dev/null
+++ b/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/__tests__/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.spec.ts
@@ -0,0 +1,112 @@
+import { createEmptyFlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/constant/create-empty-flat-entity-maps.constant';
+import { FlatEntityMapsExceptionCode } from 'src/engine/metadata-modules/flat-entity/exceptions/flat-entity-maps.exception';
+import { type SyncableFlatEntity } from 'src/engine/metadata-modules/flat-entity/types/flat-entity-from.type';
+import { type FlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/types/flat-entity-maps.type';
+import { addFlatEntityToFlatEntityMapsThroughMutationOrThrow } from 'src/engine/workspace-manager/workspace-migration/utils/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util';
+
+const createMaps = (): FlatEntityMaps<SyncableFlatEntity> =>
+  createEmptyFlatEntityMaps();
+
+const makeFlatEntity = (
+  id: string,
+  universalIdentifier: string,
+  applicationId?: string,
+): SyncableFlatEntity =>
+  ({
+    id,
+    universalIdentifier,
+    applicationId,
+  }) as unknown as SyncableFlatEntity;
+
+describe('addFlatEntityToFlatEntityMapsThroughMutationOrThrow', () => {
+  it('should index a new entity by universalIdentifier and id', () => {
+    const flatEntityMapsToMutate = createMaps();
+    const flatEntity = makeFlatEntity('id-1', 'uid-1', 'app-1');
+
+    addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+      flatEntity,
+      flatEntityMapsToMutate,
+    });
+
+    expect(flatEntityMapsToMutate.byUniversalIdentifier['uid-1']).toBe(
+      flatEntity,
+    );
+    expect(flatEntityMapsToMutate.universalIdentifierById['id-1']).toBe(
+      'uid-1',
+    );
+    expect(
+      flatEntityMapsToMutate.universalIdentifiersByApplicationId['app-1'],
+    ).toEqual(['uid-1']);
+  });
+
+  it('should append entities that share an applicationId without duplicating', () => {
+    const flatEntityMapsToMutate = createMaps();
+
+    addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+      flatEntity: makeFlatEntity('id-1', 'uid-1', 'app-1'),
+      flatEntityMapsToMutate,
+    });
+    addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+      flatEntity: makeFlatEntity('id-2', 'uid-2', 'app-1'),
+      flatEntityMapsToMutate,
+    });
+
+    const universalIdentifiers =
+      flatEntityMapsToMutate.universalIdentifiersByApplicationId['app-1'];
+
+    expect(universalIdentifiers).toEqual(['uid-1', 'uid-2']);
+    // Distinct entities, so each universalIdentifier appears exactly once
+    expect(new Set(universalIdentifiers).size).toBe(
+      universalIdentifiers?.length,
+    );
+  });
+
+  it('should not index entities that have no applicationId', () => {
+    const flatEntityMapsToMutate = createMaps();
+
+    addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+      flatEntity: makeFlatEntity('id-1', 'uid-1'),
+      flatEntityMapsToMutate,
+    });
+
+    expect(flatEntityMapsToMutate.universalIdentifiersByApplicationId).toEqual(
+      {},
+    );
+  });
+
+  it('should throw when the universalIdentifier already exists', () => {
+    const flatEntityMapsToMutate = createMaps();
+
+    addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+      flatEntity: makeFlatEntity('id-1', 'uid-1', 'app-1'),
+      flatEntityMapsToMutate,
+    });
+
+    expect(() =>
+      addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+        flatEntity: makeFlatEntity('id-1-bis', 'uid-1', 'app-1'),
+        flatEntityMapsToMutate,
+      }),
+    ).toThrow(
+      expect.objectContaining({
+        code: FlatEntityMapsExceptionCode.ENTITY_ALREADY_EXISTS,
+      }),
+    );
+  });
+
+  it('should retain every identifier when many entities share an applicationId', () => {
+    const flatEntityMapsToMutate = createMaps();
+    const entityCount = 20_000;
+
+    for (let index = 0; index < entityCount; index++) {
+      addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+        flatEntity: makeFlatEntity(`id-${index}`, `uid-${index}`, 'app-1'),
+        flatEntityMapsToMutate,
+      });
+    }
+
+    expect(
+      flatEntityMapsToMutate.universalIdentifiersByApplicationId['app-1'],
+    ).toHaveLength(entityCount);
+  });
+});
diff --git a/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.ts b/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.ts
index 3657a1c5a4..163bbf89ee 100644
--- a/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.ts
+++ b/packages/twenty-server/src/engine/workspace-manager/workspace-migration/utils/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util.ts
@@ -46,11 +46,7 @@ export const addFlatEntityToFlatEntityMapsThroughMutationOrThrow = <
       ];
 
     if (isDefined(existingUniversalIdentifiers)) {
-      if (
-        !existingUniversalIdentifiers.includes(flatEntity.universalIdentifier)
-      ) {
-        existingUniversalIdentifiers.push(flatEntity.universalIdentifier);
-      }
+      existingUniversalIdentifiers.push(flatEntity.universalIdentifier);
     } else {
       flatEntityMapsToMutate.universalIdentifiersByApplicationId[
         flatEntity.applicationId