I have both a desktop and a laptop computer. I had already downloaded the latest version of the Macintosh operating system (OS X 10.8.2, Mountain Lion) onto my desktop machine and finally got around to doing it on my laptop. I fired up App Store, clicked on "Updates", and OKed my way thru the various screens presented, one of which was my Apple ID account. I figured I had already paid for Mountain Lion for my iMac and shouldn't have to do so again for the MacBook Pro, but that they'd need to know who I was to verify my eligibility.
The download and installation took quite awhile. (If you intend to upgrade to Mountain Lion, I recommend you fire the process up shortly before retiring for the evening. That way it'll be waiting for you in the morning.) After it was done, I figured I should check to see what else might need to be upgraded, so I checked on the "Purchases" option in App Store. Sitting at the top of the list was OS X Mountain Lion, with the "Download" button active. I checked "About this Mac" under the menu, and sure enuf, it said that I was running 10.8.2 already. I clicked on the "Update" button anyway, just to see what would happen, and it informed me that I had already installed 10.8, but did I want to continue with the download anyway? No, not really.
So I infer that what I just did was spend $20 I didn't have to spend in order to get a copy of Mountain Lion that I was already entitled to for free. (I tried checking my credit-card account on line, but if Apple has charged me that $20, the transaction hasn't cleared yet.) The moral of this story is that, when you shop the App Store, start by checking "Purchases", not "Updates".