@touche112 - also interested in the answer to the same question ;) -- going to enter the giveaway tentatively now, but will remove my entry if you'd prefer that only people who don't have the base game enter (just let me know!)

I also own the base game and would like the dlc, hopefully OP is cool with me entering, I'll remove it if not.

The site should have filtered the games people own if it has not for you then manual sync it or add it to your ignore list manually.