For quite a while I have been battling with multiple entries of the same application in the 'Open With' (right click/context) menu in Mountain Lion. The screenshot illustrates the issue with a double-entry of Numbers as an alternative application to open Excel files.
When I opened Dictionaries in the AppleScript Editor the issue became even more apparent as there were up to six entries of the same application in the list.
Querying my as ever knowledgable followers on Twitter, I have been swiftly informed that the issue sits with LaunchServices which need to be rebuild with the following Terminal command:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
Relaunch Finder via 'Force Quit' or reboot. Hat tip to Naupaka Zimmermann for the hint.
As usual with these Terminal commands: Use them carefully and at your own risk. It solved my issue, but it may not solve yours.