Not sure if they redeemed it or not.

You should be able to check that user's profile to see if he has it in his library. There is a Steam icon on his profile with a link. This doesn't guarantee that user didn't use it elsewhere, but should give you a good idea if he used it for himself.

Killing Floor was given for free since then, so you can't create a new giveaway for it anyway :/

Make a ticket requesting staff to mark it as received.

I think there is no way of telling whether the key has been used or not. You can try redeeming in the account which already has the "Killing floor", the key won't be redeemed saying you already have the game.
And as per "Killing Floor" you can't make any giveaways because it was previously given out for free. But you can give it out on Orphan key thread.

There is no way to know if they activated it or not. Even if wasn't on their profile, they could have activated it on an alternate account.
Regardless of the game being available for free, as long as you are willing to provide another key for the person using it, if the key was already used, I don't think there's anything to worry about (assuming you're not doing it for CV, there are countless options to give something away, even if it was free before).