Correction to my own comment below... @zhyrin is right. The engine will throw an error when trying to connect the same signal more than once. It will not crash the game, but will output the error to the ERRORS tab, as in:
E 0:00:01:0036 Lobby.gd:291 @ _connectControlsFxSignal(): Signal 'mouse_entered' is already connected to given callable 'Control(Lobby.gd)::_playMouseOverFx' in that object.
Right.. I made a simple test.
Added a new button, a couple functions and connected them like so:
I've got no errors. And the output was:
So, it seems like Godot4 just ignore if the same function is connected more than once for the same signal.