Amazon 4-pack question + giveaway

Hello,

I already have Payday : the heist in my steam library, and I want to offer some copies to a few friends.

I'm considering buying the 4-pack currently on sale at amazon : http://www.amazon.com/PAYDAY-The-Heist-4-Pack-Download/dp/B007ZE867S/ but I'm not really sure about how the redeem is working.

Will I get 4 different steam keys to distribute ? Or will it give me one unique key that will put 3 giftables copies in my inventory ? Both of these options are fine for me, I'm only concerned about the fact that I may not be able to redeem them at all, because I already have the game in my library. (i.e. Amazon gives me an unique key, I try to enter it on steam, Steam says that I already got the game, Key is wasted.)

Sorry for the stupid question, and my bad English, but I will be thankful if someone can answer that for me =[
I already bought some steam games on amazon, it worked fine, but never these kind of "4-packs" deals.

Anyway, here's a giveaway for you : Click here !

Good luck, and thanks in advance !