What can I do because of 'Duplicate Product Code' on Steam?
Fortunately, because you have the receipt for the game you should be able to basically follow this process with Valve/Steam Support but from a stand point of "I once had this account but don't remember what it was and can prove this game is mine" instead of "I've no longer got this email address". The rest of the process should be the same.
In the instance that you have no proof of purchase for that game, and nothing else linking you to the account, there would be nothing you could do to redeem the game/product a second time.