For reasons still unclear to me, the “Open With” in the context menu in Mountain Lion has a problem where several entries of the same application appear in the list over time. Sven Fechner shares how to fix the problem by rebuilding the LaunchServices. Fire up the terminal and type:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
As usual, run terminal commands at your own risk.