It's different when it's a package of a base game + dlc, you can enter if you own the base game. But the decision is in the hands of the giveaway creator, only they can tell you if you're allowed to enter or not for the dlc, so that's why FreeFall was asking that question. Normally when you already own the game and your account is synced you won't even see the giveaway because it's hidden. But as I said with packages like definitive editions, goty editions etc, things are different, in some cases you can enter even if you own everything in the package because SteamGifts can't detect it properly but of course you shouldn't do that, because it's not allowed.