Mac App Store claims newer version of my app is already installed

I had this problem with CoBook on Mountain Lion and nothing out there seemed to work until I found this:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user

Source: http://web.archive.org/web/20130519023616/http://www.hacktheday.com/force-reinstall-application-downloaded-from-mac-app-store

To fix this problem, you need to perform two steps.

1. Delete all instances of your app. There are likely copies hiding in your home folder in ~/Library/Developer/Xcode/DerivedData/
2. Reset the user domain in the Launch Services database with/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain user

The lsregister command manages the Launch Service database, or the database that keeps track of applications installed on your Mac and what types of files they open.

It's only necessary to reset the user domain (unlike the other answer from Mat E. that also resets the system and local domain) since your XCode DerivedData exists in your individual user directory. Only resetting this domain will prevent unnecessary warnings about running an app for the first time from reappearing.

Go to ~/Library/Developer/Xcode/DerivedData/ and delete your app's build folder.