It's not againt the rules as long as the gift creator allows it.

I think the rule whoch come closest to this is "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."
Therefore most people ask if they can enter a bundle for a DLC...