Duplicate entries in "Open With" menu in Finder even after rebuilding Launch Services
I have found a Terminal command that works in Mountain Lion. It gives the correct sequence of commands for lsregister to correct this problem:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user

This was provided by one Dr. Drang on the blog at LeanCrew.com in a post called Getting rid of Open With duplicates. It was posted on February 16, 2013.

You can read the blog post at the link above for all the details.

Presumably, until Apple fixes this known bug, it may be necessary to re-run this command every time a new version of an app gets installed on my system, causing new duplicate items in the Open With menu.

I must admit that this string was suggested by bmike on January 12, 2013. But for some reason I overlooked it earlier. So thanks, bmike.