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Make sure you are signed into the same iTunes/App Store account as when you purchased the app, if you have more than one. Click on the Purchases icon at the top of the window in the Mac App Store application, and you'll get a list of your purchased apps, with an Install button at the right for any that aren't currently installed. I've deleted the FaceTime app and the Caffeine app, so here's what I'm offered:

When I click the Install button, the machinery sputters to life and a fresh copy is installed.