Steam Giveaways

You can try to contact him on steam or via email. If it does not work you can open a support case.

It's very possible that the very first winner redeemed the key but didn't mark as received.

that user should at least give you a reason, either on steam or in the giveaway. :|
not really sure how you can prove the key was valid to support.

if you bought the key from a bundle site, it can be possible to revoke it and get a new one.

sadly you dont know, if the key was used by the first winner. he can have sold the key or activated in an other account (friends account for example)...
so probably your key wasnt working anymore after you rerolled.
if you write in your own giveaway, the winner probably wont see it. the winner wont get a notification, that you wrote something in your own giveaway. you can write in one of his giveaways (as long as he did some public giveaways) or add him on steam to ask him or you can also try to write him an email. the "not received" can later still changed into a "received"^^
first of all, you should try to talk with him...

if you dont get an answere, i would write to the support to delete the giveaway (normaly you would need the winner to agree, but if he dont respond an any way, you might get it^^)

and last advice: if you increase the needed level, you will get less problems :(
btw:a not received looks bad, but you wont get any punishment (you just dont get the cv for the game^^)

Before you reroll, check the winner's history. If they have unactivated keys in their semi-recent history, don't open a reroll case but a request feedback one based on the user's history.

Also, if you are giving away a key from a recent bundle, and you can actually make contact with the winner, ask them for a screenshot of the duplicate key message and contact the site you got the key from. Ask them to revoke the old one and give you a new.

In your current case, maybe the only thing you can do is to ask support to delete the giveaway. (They will try to contact the winner for approval if you cannot manage to do it. The winner has to say it is okay to remove the giveaway.) One winner ran away with the key, chalk it up as a loss and lesson, and move on. At least your stats remain spotless with 0 unsent.

Also after a week of waiting the winner to mark it as received, untick the key so it doesn't show to him anymore and go check his steam account if he has the game, because at that point you can make a support ticket based on request received feedback.
This just in case he use it and just didn't mark it.

The current winner, despite having a not-so-good ratio, is compliant with the rules.
I suspect one of the two re-rolled winners might have something to do with the key disappearance...
You could try contacting the key vendor (Indie Gala, right?) and ask them to invalidate the key and issue you a new one.