I have worked out why I am not seeing the duplicate materials. I have the Materials->Menu->Content Filter->Show Reference Materials unticked. I have traced through the import process, and cannot see how Rhino would think these are reference materials and not normal materials. Regardless, I can see it would be a problem (although you might also want each imported car to have a DIFFERENT color, in which case it's NOT a problem). The Rhino Import process is not firing a "Material Created" event - so something is not right. I have raised the question in the McNeel forums - http://discourse.mcneel.com/t/importing-a-3dm-results-in-duplicate-rdk-materials/31969
Paul