How do I remove duplicate "Open With" context menu items in Finder.app?
Since this problem happens at System Level, I'm not sure if these steps will do any difference.

I assume that, like you've said, you've rebuilt your LaunchDatabase. Just in case: 



You can try the Command Line version of the above (In a terminal):

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


Additionally, Default Apps is a nice and free System Preference pane to control default apps (based upon extension, type, etc.) You can pretty much see and associate everything from there. It should at least give you a hint whether the problem is in the association or the finder is doing something funky. 

When did this start happening? Did you install something? I see you have Xcode 4, did you get it from the App Store or you have a Developer Profile and got it via direct download? Was this happening before? Does this happen with every type of file or only some specific ones?