My giveaway has taken an unpleasant turn

today I made a giveaway I ended up recieving the key for as part of the greenlight process. I have kept the key to myself in a notepad file on my computer and have not used it. upon seeing who won the giveaway, i sent him the key. He replied to me saying that the key was activated somewhere else. i have not given this key to anyone or activated it myself. i dont know if he is trying to scam me into giving him more than one key and i dont know how to test whether the key is actually activated somewhere. is there some way i can check if the key is active or not without putting it in my own steam account? i have been honest with my giveaways in the past, even i havent made too many of them yet. i have already created a support ticket but im just checking in case you, people of the forums, might have a solution to my problem. thank you all in advance!

(ps if you are reading this, the person who won the giveaway, dont take offense, i am not accusing you, i am just covering my options here)