Controller mapping is broken on the standalone windows release. When trying to select single player my controller is mapped for both player one and player two simultaneously.
I updated the game to the latest version, tried to see if it fixes the problem, on my side I don't have this problem and no one else has reported it so it seems specific to your pc or your gamepad which makes it difficult for me to do anything sorry! Maybe try another controller or try the game on another pc if you can.