Thank you.

But there is a problem with the key. Says another account has activated it. Could you please double check this was the correct one?

8 years ago

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Received the key with the other hand (didn't remember that I already have this game, steam reported it), so the key should fit. Sorry for my English.

8 years ago

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Checked again, on account with this game. The key is valid. Please put a note on receiving a key

8 years ago

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Steam says the key is not valid. When you checked it on your account that had this game, it tried to install it because you already had the game.

If you were to try on an account that does not have the game, you'd get my invalid key message.

The only way forward for you now is to go buy another key for this game and replace the bad key with the new one. G2A.com is pretty cheap for this.