Its me MARIO another time, no my name is not mario O.ò [Question][Important]

Pretty much, there is no way to see if a key is a duplicate or not before sending it.

But, where did you get the key from, where did you save the key?

It is a possibility that the user put the key on a 2nd account and told you it was a duplicate, so beware, but once again, where did you get the key from?

No, non cè una soluzione. Non cè nessun modo di vedere se una key è stata usata oppure no.
L'unica cosa che puoi fare è comprare da siti sicuri ( ti consiglio greenmangaming o steam stesso) e creare giveaway con queste key.
Spero di averti aiutato :)

No, there is no solution. It's your burden to keep your keys safe and make sure they're not used.

Also, the guy could very well have lied, just pray you don't land on that kind of guy.

I just got one case, recently, where one of my winners told me I had given him a duplicate key. After a quick verification, I noticed I had accidentally put a key used for myself in the text file I store my keys in, so the guy was, without the shadow of a doubt, telling the truth.

There is no real solution other than managing your keys properly, in that case you will know and if the winner claims it's duplicate, you'll know he/she is lying.

Oh, it's you again.

No, not really. You can't tell whether a key still works or not before you put up a GA for it, although usually the source of your key may provide a clue whether it still is valid or not...

You can also look at the person who won your GA, whether he looks legit or not.

If you feel that the game key is most likely a duplicate, well, what you did was really nice. But to avoid too much loss, you can ask the person to wait for seven days, while you get the game for them from another source (for example, steam's weeklong sales, or tremorgames).

Or you if you can't afford another key, well, you'll get a not received - which you can always settle later.

EDIT: and I was kidding about the ref earlier. It means "refined metal", which is an item in Team Fortess 2.

there is no way to check if the key is working that's why i always check the winner's wins and if activated the games he won
are you sure the key was not used? did you got it from shady traders or shady reselers like g2a? did you checked if the winner activated previous wins or re-gifted them? did you checked if the user marked previous wins has not received and has the game activated on his account?
check his steam profile and make sure he didn't activated and told you that is not working (yes, there are users like that)
if you think that the winner is somehow trying to get another copy of the game report the user with proof (screenshoots from games list, steam chat, etc..) but if he is telling you the truth he deserves his "reward" indeed but if you think you can't get him the game you might wanna contact the support and ask for a solution like delete the giveaway (the delete giveaway button always keeps after it's over but i'm not sure if works)

Ty anyways for help, this site have a great community, sorry for to many question, but i join here yesterday.
TYYY