Giveaway with games I already have

you can just ask the creator of the giveaway if you can enter, he's the one who decides.

If you have the base game, you shouldn't enter GOTY/Complete packs just for the DLC as that seems rude imo.

As far as large packs/collections, I would ask the giveaway host how many is acceptable to own before entering.

For complete packs, if the creator of the giveaway says in description not to enter if you own the base game. Then don't enter. If you are talking about game collections, same thing. Usually they will say if you own 3 or more games, don't enter etc. If they don't say anything in description, feel free to enter. If they re-roll, it's their fault for not informing in the description.