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Thank you for reporting a bug to Unity. After reviewing the issue, it was determined this behavior (without any form of NetworkSceneManager configuration) is expected, since MPPM will try to mirror the scenes you have loaded. The way to handle scenarios where you have multiple scenes loaded when you first start is outlined in the Client synchronization mode (https://docs.unity3d.com/Packages/com.unity.netcode.gameobjects@2.10/manual/basics/scenemanagement/client-synchronization-mode.html#additive-client-synchronization-mode)) section of the Integrated scene management documentation. In order to get the expected behavior described, one must set the host/server's client synchronization mode to additive and enable post SceneSynchronization scene unloading on the client side. This assures any scenes already loaded on the client side will be used in place of loading new scenes while also assuring any scenes not used, on the client side, for synchronization are unloaded.
How to reproduce:
1. Open the attached "IN-136817_NetworkManagerMultiSceneBug.zip" project
2. Open the "SampleScene"
3. Drag "SampleScene 1", "SampleScene 2" and "SampleScene 3" into the Hierarchy
4. Open the Multiplayer Play Mode window (Window -> Multiplayer -> Multiplayer Play Mode)
5. In the opened window, check the checkbox for Player 2
6. Enter Play mode
7. In the opened "Player 2 (Client and Server)" window, click on the layout (top right)
8. In the layout, enable Hierarchy and Inspector and click Apply
9. In the same window's Hierarchy, select the "NetworkManager" GameObject (under "DontDestroyOnLoad")
10. In the Inspector, under the "Network Manager" component, click "Start Host"
11. Return to the main Unity Editor window
12. In the Unity Editor Hierarchy, select "NetworkManager" GameObject (under "DontDestroyOnLoad")
13. In the Inspector, under the "Network Manager" component, click "Start Client"
14. Observe the Hierarchy
Actual result: "SampleScene 1", "SampleScene 2" and "SampleScene 3" are loaded twice in the Hierarchy
Expected result: Scenes are not loaded twice in the Hierarchy
Reproducible with: 1.8.1 (6000.0.0b11) 2.10.0 (6000.0.70f1, 6000.3.11f1, 6000.4.0b12, 6000.5.0a8)
Reproducible on: Windows 11
Not reproducible on: No other environments tested
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