sry i am pretty sure the code is valide, i already got the game and it was from a humble bundled i did not send it to a friend.
Unfortunately there is no way to pre check if a code is already used if you already own the game. You can see this link there http://www.steamgifts.com/discussion/4tBtm/how-to-check-steam-key . But like i said i am pretty sure that the code is valide and may you did an mistake with copy paste, but if this is not the case i am very sorry cause i don´t dave anymore codes of this game