Resintalling Twitterrific from the Mac App Store (Mac)

I'm trying to reinstall the app from the Mac App Store, but it says it's already installed. What should I do?

If the Mac App Store sees a backup copy of an app anywhere on your system, it may think the app is still installed. Make sure you disconnect any backup drives you have connected to your computer, including any Time Machine backups. Then relaunch the Mac App Store and try again.