won a game but the key is duplicate !

1. Contact the creator (try posting on the GA or adding him/her on steam)
2. Tell them key is duplicate
3. If they say they'll provide a new one, wait
4. If they say they will not provide a new one, cry and wait 7 days, then mark as "not received"

ask the giveaway creator for a new key. if he can't provide a new one within 7 days, mark it as not received. sorry that your first giveaway is a disappointment. :)

ok thanks i will try to talk him.

I'm new too and this pretty much always happens to me... Well it has happens about half the time. I don't know why people constantly put up fake giveaways.

The last time this happened to me, I asked the poster if he had a different key to give away, since he already took the one he tried to give me. He said, "No." So, I said, "OK". I waited 7 days, marked it as not received.
Later that day he contacted me to to request that I change the status to received. I said, "No." Then, he left negative feedback for me on my trade page (even though there was no trade) and he filed a support ticket against me (even though he had no reason). Looks like support deleted his negative feedback.

What I'm saying is... I hope it goes better for you.

Remember: Do not mark the game as recived until you get a working key.