Some icons appear twice in the Sidebar

Question

I have two MacBooks and a Mac Mini that are synchronized through Mobile Me. All times they synchronize to Mobile Me, the MacBooks (which runs Mac OS X Lion) keep showing some icons two times, in the sidebar of the Finder; this doesn't happen to the Mac Mini, which runs Mac OS X Snow Leopard.

What can I do to avoid this, apart avoiding that the MacBooks running Mac OSX Lion synchronize with Mobile Me?