Steam won't let you buy the same product twice (without it being a gift). For your particular case with the with the Potato Sack Pack containing things you already own, on the product listing it mentions:
About this game
Already own one or more of the games in this pack? Not to worry, you can give your extra copy of any game in this pack to a friend.
Extra copies are essentially the same as gifts, and you can freely give them to whomever you want. The recipient need not have a Steam account, the email Steam sends will instruct them how to make an account.
However, you are by no means guaranteed to get extra copies when you purchase a pack containing games you own; the Steam KB mentions
Extra copies are not given in most cases.
Only titles purchased in special promotions as outlined below are eligible for giftable extra copies
It lists a few (Orange Box, L4D packs, Dawn of War II), but obviously if the product page explicitly mentions so, then that would take precedence. Additionally, the cost of a pack is not prorated, regardless of you already owning some parts of it.
In order to gift the spare copies, in the Steam client go to Games >> Manage Gifts and Guest Passes... There it will list all the extra copies you have to give, and gifts you've given that have been redeemed (or not).