I had this happen to me twice. The first time, it turned out that the View I was trying to Duplicate had a Callout on it that had been Duplicated as Dependent. Therefore, it couldn't duplicate the Parent, because it would have also copied the Callout. That one I made the Callout Independent, and then I could Duplicate what I needed.
The second one - for some reason I was able to do a DasD of another view that was under the main & it let me Duplicate it. Kind of the opposite of what you're supposed to be able to do, but it worked.
I think # 1 is the real solution, though.