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