When you activate one key wich game you've already has, the Steam app shows the name when you choose install it. Please, do this (proceed the install, but no need to complete - cancel the process if you want) and you'll see the name game. The Athopiu maybe was installed before and you don't remeber or was a key from random key giveaway (like giveaway.ru type) and you didn't notice.
Once you've already have seen the game key, I can't guarantee, to the other person, that it was not used it.