look to the right of (20p) and you should see some symbols up there. It's the Steam symbol, a magnifying glass symbol, and after that, a symbol to manually block a particular DLC from showing up in Steamgifts ever again.
Please do it; I had to reroll my soundtrack DLC gift after a person signed up for the raffle and already had the soundtrack to Hacker Reboot. There's currently no way in Steamgifts to filter DLC you already have - it's limited to games you already have, or DLC that's useless for you because you don't have the base game. Like I said, it doesn't filter the DLC you already have. '
I'm currently on a 2nd reroll for a different game, and it's tough for us chronic gifters to show up every day dealing with common problems.