What if I join a giveaway that offers more than one game?

I would suggest that you dont enter those giveaways.

I suggest not entering in the first place if you don't intend to get rerolled. There have been cases of such.

It varies. Some people are alright with it, but others are not. It's better to ask the giveaway creator before entering.

If you already own one of the games that's ok. but if you own most of the games in that bundle/franchise it wouldn't be fair to others who don't have any of those games.

Sometimes the gifter will write in their comment that you can enter if you have less than a certain number of games from the bundle. If not, then either ask the creator if its ok or don't enter is my suggestion. One game seems to be usually ok to most people depending on the size of the bundle (but still ask).

my understanding of it anyway is:
(part 1 is much less solid of an understanding than part 2)

it is not ok to regift ever. not any part of a bundle you were gifted. you have to register and use all of your winnings. I guess if you already own it and its separate keys you'd just give the extra back to the gifter? thats what i'd do anyway(it seems like a jerk thing to take it all and then redistribute) generally its a single key anyway, I can't think off hand of any steam pack that can be split up.

2 if you already have the game the gifter can decide not to send it and reroll. (that isn't a custom rule and it doesn't need to be stated in the description or preapproved, it can and does apply in public)
So unless they go out of their way to say its ok to enter while owning part of it I'd save some tears and just assume you can't. it doesn't matter how much or little of the pack you have, any amount will do, thats up to the gifter to decide.

Usually a collection or a bundle with more than one game will come as a unique gift or as a unique key, so you can't take a game and gift it to someone, but that copy would be wasted in the process of redeeming the others (like what happens when you redeem a bundle..). So not really a general case, if it happens to you (and keys are separate), you should probably contact support or just leave the key to the creator.
Hopefully the creator will write in the description if you can enter on said giveaway, if you need only some of the games, or if you cannot enter anyway. I think from default you can enter every public giveaway is listed on the main page (apart from update issues).

The rules allow for you to enter - they also allow for the giveaway creator to reroll if you own anything from the giveaway.

About giving away extra copies; most of the time there will be no extra copies to worry about, so don't do that :p

Some times the giveaway creator explicitly warns owners of part of a pack to not enter or risk a reroll. It's definitely within their right, but what's more important, you're denying someone else a chance to to get more out of the giveaway.

Unless you're missing a substantial amount of a pack (~75%+), I'd say it is a bit selfish to enter anyway. There are exceptions, especially some expensive DLCs, but you have to use your own discretion and probably ask the creator of the giveaway for permission, to have your bases covered.