If the winner owns one of the games you'd ask support for a re-roll if you want. Generally helpful to put in the description that only those with none of the games may enter to discourage people from entering so it is less likely to need a re-roll. Some allow it based on a portion of the bundle, say if you only own 1 of the 3 games, or perhaps if you own 1 of the games which were free, or own the base game but none of the DLCs which may cost more than the game. Essentially it is up to you to decide if you want to allow it or not.

From the FAQ

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.