Question conserning Bundle Giveaways...

Can't enter if owned. Apart from some special cases that don't get detected.

They can enter, but it is your choice. It is best if you specify who you want to enter in the giveaway description, otherwise you will get a lot of comments from users asking if they can enter if they own certain games.

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.

Usually it is nice to ask if it's okay to enter for a part of the bundle (o when talking about GOTYs, to get the dlcs while having the base game). You can specify in the giveaway description if users can or can't and reroll accordingly.

You have the right to reroll any and all giveaways in which the giveaway entrant owns at least one item in the bundle, no matter what was stated or 'agreed upon' beforehand. Obviously, it is a lousy move not to state beforehand what your conditions will actually be, though.

The giveaway entrant is always allowed to enter, and you are always allowed to reroll. If you choose to send the game(s) and he activates the entire bundle as one, he has the right to mark the complete bundle as received and no rule breaking has been done. The only exception would be if he already owned the entire bundle, but in this case the bundle key should be rejected by Steam and thus it would be considered fake feedback (or regifting) if he marked as received.