Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/Resources/Scenes/MainMenu.unity” Scene
3. In Multiplayer Play Mode window, tick Player 2 (Window > Multiplayer > Multiplayer Play Mode)
4. Enter the Play Mode
5. In Player 2, observe the Console

Expected result: No Errors exist
Actual result: Error “The same field name is serialized multiple times in the class or its parent class. This is not supported: Base(AnticipatedNetworkTransform) m_NetworkManager“ is present

Reproducible with: 2.6.0 (6000.2.14f1), 2.7.0 (6000.0.63f1, 6000.2.14f1, 6000.3.0f1, 6000.4.0a5, 6000.5.0a2)

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested

Notes:
- Error sometimes appears in main Player’s Console
- User mentioned this Error is sometimes present without using “Multiplayer Play Mode“
- 100% of times reproducible when using “Multiplayer Play Mode“