Won a giveaway but the code has already been redeemed.

You can message the giveaway creator first. There's a chance they sent a wrong key. I know I have.

Contact the giveaway creator?

You should let the GA creator know that the key is already used and see if they can provide you with a working key. Communication is important and something you should do first. If they can't provide you with one then after 7 days you would mark it as not received. The GA create can request deletion of the GA, but you would need to clearly state that you agree to it, but the choice is up to you, you don't have to agree to it.