If the error message "Duplicate product code" shows, it could be either that you have already used the key in a previously created account (using a different email address), or you have purchased a dealer exchanged game. However, because we cannot access data on Steam and therefore cannot examine the key, you should report, along with proof of purchase, to Steam. Click on the Steam client in the Help menu and choose "Steam Support". A website with further instructions will open. You should receive help quickly.