To reproduce:
1. Open project
2. Build and run 'offline' scene
3. When built scene is running, play scene in editor
4. In built scene click 'LAN Host'
5. In editor scene click 'LAN Client'
6. Press space in built scene (small sphere will appear)
7. Press 'Stop' in both scenes
8. In editor scene click 'LAN Host'
9. In built scene click 'LAN Client'
10. Press space in editor scene (small sphere will appear)

Expected: messages after 6th and 10th steps should be similar
Actual: after 10th step error appears 'Object has non-zero netId 4 for Sphere (UnityEngine.GameObject)
UnityEngine.Networking.NetworkServer:Spawn(GameObject, NetworkHash128)'

NOTE: if space button will be pressed more than once in 10th step, after a few clicks you will see messages, that are as expected result

Reproduced on: 5.5.1p1, p3, 5.6.0b5, b9
Not reproducible: 5.4.4p1 5.5.0a6, f2, 5.5.1f1 5.6.0b3, b4