Players Can Overwrite Each Other’s Moves

Title:
Players Can Overwrite Existing Moves

Description:
Currently, the game does not properly prevent players from overwriting each other's moves. Although there's a check to see if a space is already occupied, it does not handle the overwriting issue correctly, leading to moves being replaced.

Steps to Reproduce:

Start the game.
Player 1 makes a move.
Player 2 selects the same spot.
Observe that Player 2 is allowed to overwrite Player 1's move.
Expected Behavior:
Players should not be allowed to overwrite moves in occupied cells.

Suggested Fix:
Add a stronger check to ensure that once a move is made in a square, no player can overwrite it.