About SG rules:

What are the rules for entering to win Steam packs if I currently own a number of the games contained within?
We ask users not to enter giveaways for packages if they already own the majority of the games. This allows for winners that will receive the most value out of their gift. In the end, the giveaway creator has the final say, and if they believe the winner owns too many of the packaged games, we'll allow them to request a new winner.

= It's a GA creator decision to allow others to enter for DLC (if they have base game or game+ other DLC's).
This is only your decision to allow / disallow.