May I enter a giveaway for a bundle if I already own one of the games?

It's not actually against the rules, but I'm sure you're too nice of a person to try to win a bundle you already own part of when there's other people entering the giveaway who don't have any of the games in it, right?

(But seriously, go ahead, nobody actually cares.)

If you only have like 1 or 2 of the games, I don't see a problem with it. unless its a 3 game bundle or something.

It depends on the contributor.
Some people don't mind, but some do.