diff --git a/storage/manager_memory.go b/storage/manager_memory.go
index 64621a84d..d12f51c3b 100644
--- a/storage/manager_memory.go
+++ b/storage/manager_memory.go
@@ -50,9 +50,21 @@ func (m *MemoryManager) Upsert(_ context.Context, collection, key string, value
 
 	// no need to evaluate, just create collection if necessary.
 	m.collection(collection)
+
 	m.Lock()
-	m.items[collection] = append(m.items[collection], memoryItem{Key: key, Data: b.Bytes()})
-	m.Unlock()
+	defer m.Unlock()
+
+	var found bool
+	for k, i := range m.items[collection] {
+		if i.Key == key {
+			m.items[collection][k].Data = b.Bytes()
+			found = true
+			break
+		}
+	}
+	if !found {
+		m.items[collection] = append(m.items[collection], memoryItem{Key: key, Data: b.Bytes()})
+	}
 
 	return nil
 }
diff --git a/storage/manager_test.go b/storage/manager_test.go
index e24126df8..ea81f36b6 100644
--- a/storage/manager_test.go
+++ b/storage/manager_test.go
@@ -89,6 +89,21 @@ func TestMemoryManager(t *testing.T) {
 				assert.EqualValues(t, 1234, vs)
 			})
 
+			t.Run("case=upsert", func(t *testing.T) {
+				var v string
+				require.NoError(t, m.Upsert(ctx, "test-upsert", "foo", "bar"))
+				require.NoError(t, m.Get(ctx, "test-upsert", "foo", &v))
+				assert.Equal(t, "bar", v)
+
+				require.NoError(t, m.Upsert(ctx, "test-upsert", "foo", "baz"))
+				require.NoError(t, m.Get(ctx, "test-upsert", "foo", &v))
+				assert.Equal(t, "baz", v)
+
+				var vs []string
+				require.NoError(t, m.List(ctx, "test-upsert", &vs, 10, 0))
+				assert.Equal(t, 1, len(vs))
+			})
+
 			t.Run("case=list", func(t *testing.T) {
 				for i := 0; i < 10; i++ {
 					require.NoError(t, m.Upsert(ctx, "test-list", fmt.Sprintf("list-%d", i), i))