Well, I don't know what I do.

ask once more. give irrefutable proof of the duplicate key. tell the guy you have to give not received.

Send him also screenshots that you don't have that game and tell that you will need to report that on steamgifts (still trying to be polite). If he won't help you after about a week you can report him, but you won't get a game anyway. I don't think there is other option. :)

I would mark it as unreceived. You can always change it back if the gifter comes through later.

Ask once more, linking to this discussion thread. If you don't get a response, then mark it as Not Received. You can always change it later if things get worked out. I hope this gets resolved for you!

 I've contacted him, but in a rude way     

To be honest, I am not suprised he doesn't answer anymore. Mark it as not received, I suppose the giveaway creator will happily take it as that.
Otherwise apologize and learn some basic social skills for the next occasion you want something from someone.

Honestly, the only person loosing something is him. Marked as unreceived and wait for his whining if he just so happens to be that kind of person.

Have you tried the key more than once or asked the giveaway creator to make sure you have the key copied properly? I've tried activating keys, only to find that I'd mis-typed something or left off a letter :p

rule #1, dont bother with support here.

fake giveaways happen here all the time, your only recourse is to mark it as not received, and forget about it. its unfortunate, and i hope it wasn't a really good game you really wanted, but thats all you can do if the gifter wont help. You've done your best to ask him to give you a legitimate key, the only thing you can do now is mark not-received.

I don't really have much else to contribute that others haven't said already. If it has been a week, I would try to contact him again, give him 3 days to respond, then mark as "Not Received" if he doesn't respond. If some more time passes with nothing coming up I would report him for "Misleading Giveaway" or maybe "Other" if this doesn't exactly fall into that.

But yes, showing the game as missing from your library is a good idea, he can even check that himself so long as your profile is public. Maybe a short video of you entering the key and it not working so there is no possible way for you to be using a different key instead of the one he gave you.

Mark it as not received man, rules are rules. Also try again the key, it happened to me before with a won HB key that appeared as already claimed, I panicked... then retried, same thing. I wrote an email to the sender explaining everything (took me 5min maybe).

He replied that it was weird and all, I retried the key and VOILA, I got the game added to my account, I activated the key, I confirmed the gift. Everyone lived happily ever after.

Mark as not received or you'll get temporary banned for "not activating the game". Don't report him since it is not viable reason to report.

Mark as not received already, notify creator about your action, and tell him that he can correct this situation anytime he wants by sending proper key.

You can still change it from not received to received later if needed.

Had similar situation

it happens. sometimes with tons of games you lose track.
anyway, marked as you did and move on. not like you lost something. he lost a GA slot (if he gave 300 it won't bother him)

All I can say is that you won already enough games, so meh I dont care ^^

I was on the other end a while back, but with a gift link not arriving to the winner's account. When I tried to see if the key was activated, Humble Bundle said it was already "Claimed". Thankfully, that just meant the gift link had been sent. They sent me the page for it and I gave it to the winner through chat, since the email just didn't arrive for some reason.

I think that, since he was rude with you and did not at least try to see what was up, you should just mark it as 'unreceived'. You can still change that to 'received' later, if he does decide to try and sort it out. Try the key again, as others have suggested and if it doesn't work after a few times, then that's it for now.

The winner in mine was kind enough to wait more than 7 days, but I also kept in contact with him so that he would not feel abandoned or tricked. I know there are a lot of folks who try to trick others, but being rude doesn't help, since you might insult a legitimate user who just wants their game. And I would have accepted a 'not received' had it not been resolved, since as a giveaway creator, I am responsible for getting the game to the winner. That's just how things are.

It's always murky territory. You could very well be a regifter or you could have given the key to a friend and be trying to get another copy out of him. On the other hand, they could be trying to give away bad keys just to level up and trying to dupe you. There's no way for either side or outsiders to know, so just follow proper procedure and it's settled.