Beginning in K-3D 0.7, you can enable/disable rendering of objects on a per-render-engine basis. What's happening here is that when you duplicate your object, it isn't being added to the list of visible objects in your render engine. This is an error-of-omission in the UI code, I've added a bug report to the tracker.
In the meantime, you can do this manually. After duplicating your objects, open the properties for your render engine ("RenderMan Engine" in the default document), and edit the "Visible Nodes" property to turn-on visibility for your duplicated objects.
Cheers,
Tim