How do I remove duplicate "Open With" context menu items in Finder.app?

For whatever reason, the "Open With" context menu in Finder is **always** listing every app four times (exactly). I've also used Onyx.app to clear user and system caches, but I'm still left with four repetitions of each app under the "Open With" service. I have restarted the system a few times just to be sure it was not some temporary corruption of the list. I'm running OS X 10.6.6 on a 2011 17" MacBook Pro with a recent fresh install of OS X (i.e. I didn't use Migration Assistant.app or a Time Machine backup). - no external or network drives mounted (including any time machine destinations) - Same list of duplicate apps if I create a new user account How can I fix this so one app shows?