How do delete stuck download of Xcode 5 from app store?
It may help to manually remove the Launchpad databases from your home directory. The databases should be located at ~/Library/Application Support/Dock...The files may have UUID names (long string of hex chars)...Removing those files, performing a logout/login (or a reboot) should allow those files to be rebuilt from the exiting applications in /Applications.

You can get into the Library folder by holding down the "Option" key on the keyboard, clicking on the "Go" menu (when you're in the Finder), and choosing "Library".