What happens if a new winner is picked for a giveaway but previous one has used/sold the key?

PS. Sorry if this is no the appropriate part of the forum.

You (or support) should be able to see if the key is revealed.

You can see if the key was revealed by the winner or not - if it was revealed you should NOT reroll cuz you can not be sure if it was used. [the NOT is personal opinion]

Ah, I see, there is a little green eye next to the key if the key has been revealed. OK, that helps a lot. Thank you both!
And indeed, no point in re-rolling if the key is reveled, I agree.

Only time I rerolled after the key was viewed, the key was used indeed (and by someone who had already won the same game and not activated it back then :x)
Only time I didn't reroll after the key way viewed (I think I gave it to a friend or sth), the key was actually not used. Still, winners should at least have the decency to check if they don't already own the game before revealing, that's just a simple click that will make everyone's life easier.

Sometimes winner can't know in advance whether he can redeem the game or not. It's very often the case with DLCs where people never check if they own it before they try to redeem a key and get already owned message. This one is possible to check before redeeming, but e.g. region locked keys or those with mixed subIDs might not be, especially if you simply couldn't know about it in advance.

Revealed key doesn't equal dupe one, I wouldn't trust a random guy in L0 giveaway, but I never had any "incident" of this sort on my usual L2+ despite of many rerolls that happened regardless of key being revealed, majority taking place before this feature was even implemented. I mean, you don't have a huge choice either. You can't "force" winner to take your key, at best you can ask for giveaway deletion but that doesn't solve anything in terms of key being valid. If anything happens, I'm quite sure Support can resolve it, including appropriate punishment for using a key without a word, including forcing received feedback back on him (probably only cg can manually set winner though).

if the winner revealed the key, send a ticket asking for feedback. not worth risking dealing with a used key.
if the key wasn't revealed, unmark the send key checkbox and request a new winner.

As Mully has said, please send a ticket to SG moderators asking them to help you chase for the feedback for the unresponsive giveaway winner. If they don't respond, then SG moderators can deal with it. If the person deliberately does not provide feedback SG mods can impose a punishment for not responding. Don't make another giveaway as if they have seen the key, its highly likely the key has been used or sold. Also if you can, you can leave a message on their steam profile page requesting feedback as some people only come onto SG occasionally. Hope this helps. The few times I have had this issue, putting a message on their steam profile page usually resolves it.