Excess copies of games

I think so. The only exception here would be Team Fortress 2.

Generally, it's a little bit of a dick move to enter for packages where you have some of the items. But anyways, it's all about the invidiual products on Steam, so you should check the entry out on the Steam store. If a package says you'll get the bonus, then you do, just like if you bought it on Steam yourself. Your specific example, would get you extra copies of HL2 and HL episode 1, which you can see if you check out the orange box's store page (scroll down).

I don't think you can. At least in games which are activated as a bulk. For instance if you activate a copy of Orange Box and already own Portal or Counter Strike: Source + Garry's already own Counter Strike: Source you will not get extra copies of Portal or CS: Source as giftable copies. This applies to keys or gifts I think. I was recently gifted a copy of Counter STrike + Condition Zero and although I already owned Counter Strike I did not get it as a giftable copy afterwards.

I think it goes on a game by game basis. I think certain games will give you a gift-able copy while most will not. I don't know which games will and which games won't however. I know with the Humble bundle a few people who already owned cogs received a gift-able copy. I was not so lucky though as I did not get an extra copy and I previously owned cogs.

It is as Corpekata says a 'not nice move' to enter for a package you already own most of. However it is not against Steamgifts rules.

Yes, and if you win any valve pack you can gift the others to gamers and make them happy! SPREAD THE LOVE!