diff --git a/packages/entities/src/lib/update.mutation.spec.ts b/packages/entities/src/lib/update.mutation.spec.ts
index acb5a643..c8b43bb7 100644
--- a/packages/entities/src/lib/update.mutation.spec.ts
+++ b/packages/entities/src/lib/update.mutation.spec.ts
@@ -259,6 +259,19 @@ describe('update', () => {
 
       expect(ids).toBe(sameIds);
     });
+
+    it('should be able to handle duplicate ids', () => {
+      const store = createEntitiesStore();
+      store.update(
+        upsertEntities([
+          { id: 1, completed: true },
+          { id: 1, completed: false },
+        ]),
+      );
+      const { ids, entities } = store.getValue();
+      expect(ids).toEqual([1]);
+      expect(entities).toEqual({ 1: { id: 1, completed: false } });
+    });
   });
 
   describe('UpdateEntitiesIds', () => {
diff --git a/packages/entities/src/lib/update.mutation.ts b/packages/entities/src/lib/update.mutation.ts
index ab54e7bc..8fec0275 100644
--- a/packages/entities/src/lib/update.mutation.ts
+++ b/packages/entities/src/lib/update.mutation.ts
@@ -228,8 +228,9 @@ export function upsertEntities<
 
     for (const entity of entitiesArray) {
       const id: getIdType<S, Ref> = entity[idKey];
-      // if entity exists, merge update, else add
-      if (hasEntity(id, options)(state)) {
+      if (asObject[id]) {
+        asObject[id] = { ...asObject[id], ...entity };
+      } else if (hasEntity(id, options)(state)) {
         asObject[id] = { ...state[entitiesKey][id], ...entity };
         updatedEntitiesId.push(id);
       } else {