Remove 'Open With' Duplicates in OS X
Frustrated by Pixelmator.app appearing in Finder’s Open With menu five times? Does your Open With menu look anything like this?
Time to rebuild the launch services database:
For OS X >= 10.5
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
For OS X 10.3 – 10.4
/System/Library/Frameworks/ApplicationServices.framework/\Frameworks/LaunchServices.framework/Support/lsregister \-kill -r -domain local -domain system -domain user