what can i do if i got duplicate key

report to internet police

You can either return it to the gifter for a reroll (the right thing to do) or give it to a friend or something.

wait for 1 week and mark as not received.

Did you try contacting the giveaway creator? I too got a duplicate key myself when I won garry's incident, so I immediately notified the giveaway creator. He apologised to me right away and said he will procure a new working key as soon as possible. I waited a few days and he gave me a legitimate key.

Mark as not received, that is the only thing you can do. And of course, contact the GA host.

You didn't mark it yet as received right? Don't ever do that before you have used it. Otherwise they just could do another giveaway with that key and you wouldn't get a thing...

What game was it btw? :)

Talk to the giveaway creator. Hopefully they can sort you out with a working key.

Don't use it. Problem solved.

cry

try it one more time but with your eyes closed

first of all talk with the giveaway creator tell him that not working
if he doesn't answer you up to 7 days mark this as not recive

i just want to know if it's have anotherways

so appreciate all your kindness

with such great humor base on insult

1) Check to make sure you entered the key correctly. (People make mistakes.)
2) Tell the giver that the key is a duplicate/invalid. This is best done right when you get the key.
3) If the giver cannot help or refuses to do so, wait seven days and mark as "Not Received."

Rage. Then tell him you want to trade it back for a working key.