I've not seen the term used before, but I have a couple of possibilities.
First, it could just be that they don't like the idea behind the clue. (ie it's a problem with the fundamental premise - ie the root - of the clue). If this is the case, it's a weird way of saying it.
Second, which I think more likely, is that in a double-def clue, you're using two definitions come from the same root and therefore it's not actually a double definition but a 'single definition twice'. This can also happen in clues where you get normal wordplay, of course, because you could define part of a clue with a different word from the same root. Of course, now I'm trying to think of examples of this and failing miserably, but hopefully you get the idea.
H/S