Hello,
I checked in my account, and it shows me: "That me already have the game", not that, that it is already used.
There are different message in case it is already used in an another account, or you have the game..
I got this key in a promotion, but I had this game from 01.01.2016.

Can you send me a screen opn Steam (if possibel in English) what kind of message you got when you try to activate the code?

I uploaded an another key, you can try this too, this must work (as the other too, I don't know why that was not worked).