So people can just scam the system without any consequences?

Calling out is not allowed, remove the URL to the guy before mod sees it and you get suspended.

Have you tried sending "request received feedback" ticket on your giveaway? If support didn't leave any response, then you most likely sent user ticket instead, those aren't receiving a response.

No calling out so better to remove that profile. Also a lesson, that's what you get for doing level 0 giveaways, bots and scammers.

You should remove the user. You might get banned for that. Calling out is against the rules.
Sorry.. for you.

you should remove the profil since calling out is against the rule

for future giveaway you should check if people already have this game or not. Or make invite only to remove some of the bot .

With the given information it's still pretty easy to find him
did he told you why it is marked at not received?

If you are actually American and this is about Dead Island, then there is a 99.99% chance he could not activate it. Deep Silver uses an NA lock and if you try to activate a key from an American in Europe, you get a region-restriction message. Humble even states rather clearly when generating the gift link that it is a non-EU key.

I noticed that he owned the game already so I became a little suspicious.

Wrong. You gave away Dead Island Definitive Edition and he owns Dead Island. Not the same game.

I know the key was redeemed because it now gets hidden when I check "Hide redeemed keys" on my humblebundle keys page

There is, unfortunately, no way to know if a key has been used unless you try using it yourself, if you don't already own the game (which would of course consume it in the process). The "hide redeemed keys" function just hides keys that you revealed but provide no indication as to whether or not they were actually used. Thanks Valve :/

Call me paranoid but seems like an alt account

We'd like to avoid witch hunts and it's really identify the user, so I'll close the thread.

Comments are disabled on user reports, so if you have an issue with a giveaway you created, there are more appropriate categories to use, like a feedback or deletion request.
You can try to contact the winner as well to see what happened. He doesn't actually own the game, just the older version of it so there's probably some other issue.