I struggled to find anything similar. The closest occurrence was when I actually had two different versions of a movie, or where two different file types were imported into MC because of the file type in Auto Import.
But then I found one. However, when I drilled into the details the duplication disappeared. Then when I backed out of the View, refreshed the page, changed Views, and went back in, the duplication was still gone.
I even closed MC and re-opened it. The duplication was still gone.
So it seems to be a transient indexing thing. A bit hard to fix I would guess. It is possible that for the example I found I used to have two copies, had deleted one, and the indexing hadn't been updated, and wasn't until I looked at the detailed View for the Genre. Maybe that is fixable, but a fix might slow Theatre View.
I tested on a Client connected to my HTPC MC Server.