Question about giveaways with steam key.

nope, nobody tests keys

There's no way to test it and it is definitely possible for someone to say it doesn't work and sell/trade/giveaway it themselves

to avoid those situations you should make the giveaway with conditions(using STTools) or public giveaway with high levels to enter

There is no way to test them, but if you do giveaways for special groups or put at least a lvl 2 restriction on public GAs, you should be fine :)

It's not possible to test a key. And just like you suspect, it is possible for a winner to lie about a key not working. You may have your doubts, but unfortunately you're never going to be able to prove anything - there's no way for us to discover which Steam account has activated a particular key.

Now, if you bought the key at some authorised store like Indie Gala or Humble Bundle, their support can apparently revoke a key if you suspect a winner is lying about it being used. The store will provide you a new key, and whatever account added the old key will lose the game.

Some people refuse to make "level 0" giveaways, assuming people who have never given a single thing are more likely to cause problems of that sort...

Oh, one thing, I suggest you always check your giveaway winners with http://www.sgtools.info/ before sending anything. If you find any problems, submit a re-roll request so the rule breakers don't get your key. :)

In my situation, i won a GA, and received a duplicated key, i asked the owner to check but no answer.
What am I to do now, they all might think i lied about it, but the fact that you can also give a used key for winner, that makes nonsense

most people mark received, most giveaway creators have the same concern yet here we are with hundreds every single day, you shouldnt overthink or worry

winners can claim a key doesn't work so you send another, but i don't think it's common practice here. it's also risky for cheaters since they will get several reports for the same offense and i'm sure support with notice that behavior.

also, i think you can request a key to be deactivated and get a new one in most sites (like humble bundle). :3

I've only sent keys for 95 games and counting... and I have had NO problems... Ugh, please don't jinx myself, no jinx, no jinx...
So, yea, by and large the Steamgifts community is a kick-ass, upstanding group of gamers.
(There may be the occasional evil doer, but it happens rarely)