diff --git a/src/registry/mod.rs b/src/registry/mod.rs
index 5f0c6d58..5793388e 100644
--- a/src/registry/mod.rs
+++ b/src/registry/mod.rs
@@ -293,8 +293,22 @@ impl Registry {
         let mut dummy_registry = Registry::default();
         T::create_type_info(&mut dummy_registry);
         if let Some(ty) = dummy_registry.types.remove(&*T::type_name()) {
-            self.types.extend(dummy_registry.types);
-            self.implements.extend(dummy_registry.implements);
+            // Do not overwrite existing types.
+            for (name, ty) in dummy_registry.types {
+                if !self.types.contains_key(&name) {
+                    self.types.insert(name, ty);
+                }
+            }
+
+            // Do not overwrite existing implements.
+            for (name, interfaces) in dummy_registry.implements {
+                if let Some(current_interfaces) = self.implements.get_mut(&name) {
+                    current_interfaces.extend(interfaces);
+                } else {
+                    self.implements.insert(name, interfaces);
+                }
+            }
+
             ty
         } else {
             unreachable!()
diff --git a/tests/merged_object.rs b/tests/merged_object.rs
index 47f01ee5..7ead6c4f 100644
--- a/tests/merged_object.rs
+++ b/tests/merged_object.rs
@@ -290,3 +290,49 @@ pub async fn test_merged_entity() {
         })
     );
 }
+
+#[async_std::test]
+pub async fn test_issue_316() {
+    #[derive(SimpleObject)]
+    struct Fruit {
+        id: ID,
+        name: String,
+    }
+
+    struct Query;
+
+    #[Object]
+    impl Query {
+        #[graphql(entity)]
+        async fn get_fruit(&self, id: ID) -> Fruit {
+            Fruit {
+                id,
+                name: "Apple".into(),
+            }
+        }
+    }
+
+    #[derive(Default)]
+    struct Mutation1;
+
+    #[Object]
+    impl Mutation1 {
+        async fn action1(&self) -> Fruit {
+            Fruit {
+                id: ID("hello".into()),
+                name: "Apple".into(),
+            }
+        }
+    }
+
+    #[derive(MergedObject, Default)]
+    struct Mutation(Mutation1);
+
+    // This works
+    let schema = Schema::new(Query, Mutation1, EmptySubscription);
+    assert!(schema.execute("{ _service { sdl }}").await.is_ok());
+
+    // This fails
+    let schema = Schema::new(Query, Mutation::default(), EmptySubscription);
+    assert!(schema.execute("{ _service { sdl }}").await.is_ok());
+}