Regarding bundles.

Depends on the person who raffles the game. Some people allowed users to join a complete edition of Skyrim even when they already had Skyrim in the first place.

I BELIEVE the rule is that only private giveaways can actually prevent you from entering IF the creator wishes it.

If its a public then its just common courtesy to pass unless the game you own in said bundle is of the lower end/the unpopular games.

It depends on the person giving it away, really. It usually never hurts to ask, either. If there's nothing stated on that matter, or if you don't get a reply, you're still free to enter, though the person giving it away can choose to reroll it if they feel you own too much of the bundle.

My suggestion is assume you can't enter, unless the giveaway creator explicitly says it's ok. Some people say "you may enter if you own 1 of these games" or "don't enter this giveaway just for the dlc" and so on. If you're not sure, write a question in the giveaway and hope they answer it. :) No point in being greedy... :)

It's up to the giveaway creator.