MacOS – How to remove duplicate icons from Launchpad

If you have the latest Xcode (4.3.2 at time of writing) it should be in /Applications/Xcode. You may have an older Xcode in /Developer. /Developer is now no longer used by Xcode. You should remove it using the provided script if you don't need the old version any more. Execute

sudo /Xcode4/Library/uninstall-devtools --mode=all

and authenticate. When that script completes you can remove the /Developer folder entirely but you should allow Xcode's script to take care of its somewhat "fragile" installation first.

New Xcode lives in /Applications and is much more like a conventional Mac OS X app these days. Lots more information about putting it in the dock, finding other developer apps, command line tools etc. in this other answer of mine.

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".