Reproduction steps:
1. Open the attached “IN-103953.zip” project
2. Click the dropdown next to the Play mode button
3. Choose “Server + 1C” Play mode Scenario
4. Enter Play mode
5. Observe the Console in the newly opened (Client) Editor

Expected result: No warnings, only one NetworkManager Player is spawned
Actual result: “Tryingto spawn NetworkObjectId 6 that already exists!” warning is shown and two Players are spawned

Reproducible with: 1.12.0, 2.4.0 (6000.0.50f1, 6000.1.6f1, 6000.2.0b5)
Couldn’t test with: 2022.3.62f1 (Package not supported)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Notes:
- Also reproduces in builds running locally
- Also reported in discussions: https://discussions.unity.com/t/double-spawned-networkmanager-player-objects-when-using-networkscenemanager/1650791