Already Activated scam?

Ask steam support on date of key activation. If the will be in good mood, they will tell you

Was it a key or a humble gift link? I once had problem with the gift link from a giveaway I won. When I clicked the link from my mail it said "This key page has been claimed". Later I tried to do activate from my phone. Somehow that worked and I successfully received the key.

Concidering you made two gas for same game. Are you sure you didnt enter wrong key?

I have received an already activated key from Humble Bundle once, but they were quick to replace it with a new key when I contacted them. If you sent it as a gift link the recipient would be able to simply contact Humble for a replacement key.

It may be worth NOT deleting the giveaway and having him mark it as not received. That way, if it was a scam, others can see he did it before, so he can be suspended.

That said, Distraint 2 seems like a pretty "low value" game to try to scam. Maybe just a mix up of your keys? You did make two giveaways for it.

i think i once had a problem that the winner was able to sort out with humble bundle. i had sent them a gift link.

if you are gifting a key, then usually you can't tell. typically if the winner is telling you the key didn't work they haven't activated it on the account that actually won the giveaway, and i don't know how you'd find an alt or friend's account they did activate it on for cases where that happens.

if you use humble's gift links you can visit the link yourself and it will say it's been claimed if it has. sometimes key sellers will assign you a key that's already been used. that can be because they are disorganized or dishonorable or just that somebody was trying to activate randomly-generated keys and happened to get yours.

of course if possible double-check that you actually sent the right key / giveaway link.

if its a key you can ask steam support to give you the Activation date (if its after the giveaway end) you can be sure he sold or activated on a different account . (you could ask here SG support then to make it Approved)
If its a Humble bundle link you could ask them to write to HB support first .
If the activation date was before you gave the key away (you gotta figure something out with the publisher or just delete it like the person said .)

I think a good take away is that using the gift links instead of sending keys is probably a better option

https://www.steamgifts.com/discussion/c1wQL/what-happens-if-a-user-claims-a-key-with-a-different-account-and-then-says-the-key-is-invalid