Hi. I tried to activate the key and received a message that the product code has already been registered by another user.

I have never used the site you referenced so I can't say how they source their keys, but I tried again and received the same message "Duplicate Product Code". Steam seems to think it has already been activated by another account,so that is all I have to go by. I appreciate the fact that you created a giveway, but I can't mark as "received" since I can't activate the key.