After using my PowerMac G5 awhile with Mac OS X (10.4.8) for awhile, I find out the Open With... list in the Finder has lots of duplicates. If you have the same problem, issue the following command in the terminal screen:
/LaunchServices.framework/Support/lsregister -kill -r -domain local\
-domain system -domain user
If you are typing this command, ignore the backslashes "\".
As for Mac OS X Leopard (10.5.x), the command lsregister has been moved to the other location:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user