What does "Duplicate Product Code" mean on Steam?

This error means the Steam key you're trying to activate has already been redeemed on a different Steam account. Once a key is activated on Steam, it is permanently bound to that account and cannot be used again by anyone. Steam Support cannot issue a replacement. First, click "Send email to the account owner" in the error screen — if the email arrives in your inbox, the key is already on one of your own Steam accounts. If not, contact the seller who sold you the key with your order confirmation and a screenshot of the error and request a replacement or refund.