If you get a lot of duplicates in the menu which opens if you open the context menu on a file to say „Open with“ you have to rebuild your launchservices database by issuing a little command line command. Open up the terminal application and paste the whole code from below into the terminal window, press enter and wait until the command returns:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user
Afterwards alt + right click (if secondary click activated) or ctrl + alt + click (if secondary click not activated) above the finder icon in your dock and select the restart option. Your „Open with“ menu should now be fine.
This was tested on OSX 10.8.3