If you have a file on the desktop or in a Finder window, double clicking it will open it in application. For example, double clicking a text file might open it in TextEdit. I say might, because it is possible to have several applications that can edit text files, like TextEdit, LibreOffice, Text Wrangler, Word or some other app you have installed on your Mac.
Double clicking a file opens it in the default app. If you want to open it in a different app you can right click it and select Open With. A menu is displayed that lists all the apps that you can use.
This is a great feature, but this Open With menu seems to fill up with multiple entries and items you don't want. It can end up being a bit of a mess after a while. What is needed is a way to tidy up this Open With menu and remove redundant entries.
There are several options available for trimming the Open With menu. Go to the Utilities folder and run Terminal. At the command prompt enter:
lsregister -kill -r -domain local -domain user
This is all one command that should be entered all on one line, not three commands! I've split it just to fit it on the web page. Afterwards close the Terminal window and restart the Mac or at least log off and log back in.
In this Apple support document it tells you to go to the Library folder, open Preferences and then delete com.apple.LaunchServices.plist. Looking at this file in Mountain Lion I can't see anything relevent. I think its use has changed and the page states that it's only for OS X up to 10.4. I wouldn't try it on a new Mac.
My own preference is to use Onyx. This is a free utility for the Mac that has many useful functions. Run it, click the Maintenance icon in the toolbar, select the Rebuild tab and tick LaunchServices. Click the button to execute this and afterwards restart the Mac or log out and back in. Check the Open With menu by right clicking a file and you'll see that it has been cleaned up.
Notice that I have two TextEdit.app entries in the Open With menu. One says version 1.8 and the other says 1.7.1. How come? The reason is that I have two disk partitions with OS X Lion stored on one and Mountain Lion on the other. If OS X detects a duplicate app on another partition, disk or external drive, it adds it to the Open With menu. When two or more apps are discovered and they are different versions, they appear in the menu as separate items with version numbers. So if you have cleaned up the Open With menu, but still have duplicate entries, look for the other app. It's got to be on the system somewhere, such as another disk or partition.
Affilate links follow...