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

Contracts are different and if you do try to deposit out coins/tokens which are in different then it would be still useless since it wouldnt credit out and this is the beauty of blockchain technology.