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.
For some games at least, Steam will notify you of this overlap (and loss) when you're reviewing (before) your purchase. For example, below, I already have Far Cry (1), so when I buy the Far Cry Franchise pack (includes 1-3), I'm effectively wasting that bit 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 you can either give them to someone else (or yourself, if you don't have it).