how does it work if you want certain contents of a pack...

No it is not. Certain parts of the pack will be lost and cannot be given back.

You can enter them, but you'd be kind of a dickhead.

It depends on the pack, for most packs no but if it is one of the Half-Life 2 games then yes, but only once.

Would it be rude if I enter into Humble Bundle 1 giveaway and I own World of Goo?

I take it on a case by case basis. If it's a large pack and you only have one of the games, I think it's okay, but if you have several and you just want one or two of the games in it, then it's best to leave the chance for others. As for whether you get extra copies or not, for most packs you don't.

Typically the submitter of the giveaway puts this is the description. We understand the frustration if someone won, let's say the THQ Pack, and someone already owned 7 games from the pack. We encourage members to update their description when it comes to criteria for the winner and please run it by a mod if it's a little more involved than "do not enter if you own * games from this pack".