fix #384: duplicating scene objects at runtime is now detected and and error is shown. otherwise the user could instantiate scene objects at runtime, which would then send the sceneId to the client, then the client wouldn't know which of the multiple objects to use for the sceneId, resulting in things getting out of sync.

Assets/Mirror/Runtime/NetworkIdentity.cs

@@ -150,6 +150,34 @@ internal void RemoveObserverInternal(NetworkConnection conn)
             observers?.Remove(conn.connectionId);
         }
 
+        void Awake()
+        {
+            // detect runtime sceneId duplicates, e.g. if a user tries to
+            // Instantiate a sceneId object at runtime. if we don't detect it,
+            // then the client won't know which of the two objects to use for a
+            // SpawnSceneObject message, and it's likely going to be the wrong
+            // object.
+            //
+            // This might happen if for example we have a Dungeon GameObject
+            // which contains a Skeleton monster as child, and when a player
+            // runs into the Dungeon we create a Dungeon Instance of that
+            // Dungeon, which would duplicate a scene object.
+            //
+            // see also: https://github.com/vis2k/Mirror/issues/384
+            if (Application.isPlaying && sceneId != 0)
+            {
+                if (sceneIds.TryGetValue(sceneId, out NetworkIdentity existing) && existing != this)
+                {
+                    Debug.LogError(name + "'s sceneId: " + sceneId.ToString("X") + " is already taken by: " + existing.name + ". Don't call Instantiate for NetworkIdentities that were in the scene since the beginning (aka scene objects). Otherwise the client won't know which object to use for a SpawnSceneObject message.");
+                    Destroy(gameObject);
+                }
+                else
+                {
+                    sceneIds[sceneId] = this;
+                }
+            }
+        }
+
         void OnValidate()
         {
 #if UNITY_EDITOR