The problem is a corrupt Launch Services database which can be reset by the following command in Terminal

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

and relaunching the Finder.

The solution is discussed in more detail here: TidBITS: Eliminate Duplicates in the Open With Submenu and also an older (2010) discussion here: Right-click open with... showing duplicate apps? - MacRumors Forums

I had the same problem and like Adam at Tidbits hadn't looked into it, but was happy to see a solution.

Glad to be able to contribute as I've always been on the other side.

Greg


On Jan 15, 2013, at 5:08 AM, K.-Michael Aye wrote:

Hi,

the recent updates (I'm on the nightly builds track) keep being registered as different apps in whereever MacOSX tracks if an app can deal with a certain kind of file.
So, when I do right-click->Open With, there are 4 entries for TextMate now.
How can I clean this up?

Thx,
Michael