Mac App Store Now Warns Users Before Re-buying Already Installed Apps

As Macworld reports, Apple has introduced a change to the Mac App Store showing warnings to users who attempts to buy apps, which they have already purchased from outside developers and which is already installed on their machines. The warning says:

[App name] is already installed and was not purchased from the Mac App Store

Do you want to buy [app name] again?

The report notes that such step can really help prevent users from accidentally re-buying the app in cases when  they are not aware that such application is already installed.

According to the NextWeb, versions of the existing app and the one in Mac App Store must be exactly the same. Otherwise Mac Store will allow buying the application without triggering the warning.