I mistakenly entered a giveaway for DLC I own, and I won it.

Contact creator and ask to reroll.

Edit: Giving/trading key to someone else can lead to ban from site and probably blacklist entry too.

Just contact the creator, tell him or her that you already won a copy and therefore don't need a second, and he or she will ask for a re-roll. If you want to be helpful, you can also include something in your chat session or giveaway description that says says it's "OK to re-roll." That gives Support confirmation that you have agreed to the re-roll.

This happened to me this afternoon, trying to contact the creator atm.

Or you could just whistle and pretend nothing happened. I guess because of you pc sob's I have to sat I just kidding.

I hide all DLC giveaway I already own when I see in the list, so I can't be wrong