Player spawns with multiple instances of himself :(

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Hello everyone,
I am working with Pun in Unity and I have a problem with my spawning function for 2 months now, but I can't figure out how to solve it. I am searching for the solution for 2 months and I am very sad that I can't find the solution.
My connecting behaviour works like that and is located in the "Network Manager" script:
- My player is in the Menu and presses a button
- The player connects and searches for a room if the button has been klicked
- If there is no room, he creates a new room and chooses a random scene from a list and changes the scene to that
- If there is a room, he simply joins the room

The player who creates the room spawns once (with one instance of himself) and everything is fine. But if another player joins, he spawns with multiple prefabs of himself, so there are 4 instances of himself, but he can only control/ move one of these and the other ones aren't even visible for the other players, but for the local player who spawned.
My spawning function is called in Start() in another script ("Level Manager") which is in the other scene.

I don't know if I was explaining that well, but I'll also send a recording of this problem and my two scripts.

My "Network Manager" script which is in the Menu and it's responsible for the connecting and joining (ignore the excluded code, I tried to make a game mode system, but I will do this later, after I solved my main problems): https://pastebin.com/6PgUsPzb

My "Level Manager" script which is in the other scene is responsible for spawning and other stuff which happens in the level ("Collider Occupied" just checks if a player is standing on the spawn point and it returns a boolean which is used in this script, so the player shouldn't spawn there and it also didn't work without this script):
This is the important part for the spawning: https://pastebin.com/Tm69NEq3
And this is the whole script if anyone wants to see it: https://pastebin.com/89uK60XF

Here are the recordings:
View of Player 1, who created the room (works perfectly): https://streamable.com/q4jzz
View of Player 2, who joined (doesn't work): https://streamable.com/d85x9

Please help me! I am searching for 2 months for the solution :(

Thank you very very much already in advance, I really appreciate the help :smile: