In an effort to keep users from accidentally purchasing a piece of software they might have gotten elsewhere, Apple is included a new warning system in the Mac App Store that lets buyers know when they already have the app they're trying to buy.
The new warning, picked up by MacWorld this morning, only appears when you attempt to hit the buy button from the product page of an application that's already installed on your computer. A small drop-down warning then comes up to confirm whether you want to re-purchase the software before moving on to the payment password screen.
The move represents the first real awareness of the Mac App Store with software that's been installed outside of Apple's marketplace. It also adds to the previously existing feature that would let you find and recover software you've already paid for.
As noted before, the Mac App Store does not have a return policy. Instead, Apple encourages customers to go to application developers directly for assistance, then contact its App Store customer support if that doesn't get them anywhere. Nonetheless, the company has allowed returns in certain situations. The inclusion of this new warning system is likely part of a larger effort to keep some of those returns stemming from accidental purchases at bay.