Let's assume i have created a game, and then created ERC tokens (Example swords) so the player must buy a token to be able to play the game.
What if someone creates similar tokens in his wallet and injects them into my game as if they are bought from my tokens store

Is the game you're making doesn't have a security system, how is it possible for a similar token to go into the wallet and then put into the game, this is a strange thing in my opinion on a system.
The security system of a game is equipped with anything to avoid intrusion, there is no possibility of this happening if the game that has been made has gone through the studies of people who have the ability in it, even if this happens there will be steps to bypass it or block the intruder.