Starting with Mountain Lion, I have noticed that I will often see the same program listed multiple times when right-clicking on a document to use the “Open With” functionality. For example, I just updated TextMate (which seems to be updated very regularly lately) and my Open With menu looked like this:
To fix this problem, you need to rebuild the Mac’s Launch Services Database. Open up your terminal and issue the following command:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
That should fix the issue. For some reason, I noticed that when I right-clicked on the same item after running the command, I still saw double. Clicking on another item “refreshed” the list (and returning to the original item worked correctly). It may have just taken that long for it to take effect. I did not have to run the command with elevated privileges.