i moved some icons on desktop (not in desktop folder, but desktop...) and renamed them (for example Firefox web browser to Firefox). Now i get double icons in the menu as well. Any way to turn this off or at least delete the new versions somehow...

A screenshot of menu here: http://ubuntuone.com/1ngZU6m1DvKccchvPL8aQj

Note how there are now 2 Thunderbirds and 2 Firefoxes

Ouch. You can probably use the KDE Menu Editor to remove the duplicates.