Is it allowed to create a giveaway for maybe already used keys?

make a whitelist giveaway and ask people to allow you to delete it if the key is invalid.
of course, only invite people you trust.

When creating a giveaway, you promise the game to the winner.
If you key turns out to be used already the winner might

• be a kind person and agree to the giveaway be deleted,
• be not so kind and mark the giveaway "not received" after 7 days, he/she might even report you for a fake giveaway.

From what I've seen most active forum users will agree to delete the GA in case. I would create such giveaways as invite only or group GAs.

In addition to what was already written above by Mully and Oppenh4imer, you can also skip the site giveaway system and just gift the game directly to someone who is interested in it (e.g. has it wishlisted). Obviously you won't get any CV for this, so it won't help you level up.

If you have another copy of the key you can try to GA the possibly used one and if it doesn't work, swap it for the good one.

Otherwise, no. If the winner for any reason decides he must have the game, you will get suspended for I think 3 days. You can try to persuade them by only offering the GA to one of the private groups that might kick the user out, or only to certain invited people, or only to level 10s, or only to your whitelist, but the worst case scenario applies in all.