Reproduction steps:
1. Open User attached project
2. Build a standalone player
3. In the player - click on "LAN Host(H)"
4. In the editor - run "Offline" scene
5. Player - click on "Unspawn" UI button
6. Player - click on "Stop (X)"
7. Editor - click on "Pick New Host"
8. Editor - click on "Start As Host"
9. Editor - click on "Spawn" UI button
Notice error "Object has non-zero netId X" is thrown:

Object has non-zero netId 2 for CubeInstance (UnityEngine.GameObject)
UnityEngine.Networking.NetworkServer:Spawn(GameObject, NetworkHash128)
NetworkSpawner:Spawn() (at Assets/NetworkSpawner.cs:14)
UnityEngine.EventSystems.EventSystem:Update()

Expected: no error is thrown and proper Id is given

Versions Reproduced: Unity 5.3.4f1, 5.3.4p5, 5.4.0b12, 5.4.0b14, 5.4.0b16