I won the same game...

Ask the maker of the giveaway to reroll for a new winner.

Contact creator, ask for a reroll.

Do not regift or give to someone else, you will get suspended.

mark as not received and contact the creator of that giveaway

DON"T MARK AT ALL!!!
sorry, read ttailzz comment

question: you got the key to the game? or, are you sure the first win is from someone who will deliver the game?

anyway, you should contact the GA maker and tell them to ask for a re-roll. support will approve in a few hours ;)

So how should I get in contact with the giveaway maker?

I normally leave a comment on the GA, that normally does the job. Though, you can also add them on Steam to explain it to them.

Thanks guys for the advice