Just started giving away games but running into issues

Did you sent 2 of the same key to both winners? That can happen sometimes.

That must suck... I would be highly suspicious as well. I guess you could contact humble support and see if they can do anything.

Check his profile to make sure he didn't activate the game and is trying to scam you (Highly doubt it, but worth a shot.)

If there's someone posting on his wall a lot (obviously a brother or best friend or something), you can check him, too, to make sure he's not trying to scam you for a second key for his friend.

Which one? SS HD 2? That never shows on the profile unless you play it, so I'm distrusting of this winner.

contact support and ask what to do now then..

you could also send an email to humble bundle saying that the key was dupe

Key drop it to test. If it works, the user was lying. Of course, he may have lied and traded / sold it, too.

Looks like the guy marked it received. It was the SS1 you were having issues with right?

You've got to be very careful when dealing with keys.

People of questionable morals can trade the key away while claiming it doesn't work. This is why you always record any conversation with the person receiving the key by using screenshots. Also, pirates can synthesize and use a valid key before it is even sold. In the latter case, you have to try and work it out with the seller and/or developer.

Tell him to send you a screenshot of steam saying that the key has already been used.