Sometimes I use the Sort Title field to group movies or put them in an order I like them to be. However this strange thing happened to me. I had this movie called "A Brilliant Young Mind (aka "X+Y). When I used the title "X+Y" in the Title Sort field it actually doubled the number of thumbnails (movie posters) displayed of all the movies that followed in alphabetical order. I had about 35 or so movies after "X+Y" so those titles' thumbnails showed up twice. In other words, when the Sort Title field read "X+Y" it acted as some sort of a programming operative. I had typed in X+Y without quotes like I normally do with titles.
It really had my scratching my head at first because although the thumbnails repeated, the Total Number of Files field never changed. I kept wondering why a whole bunch of thumbnails got duplicated. Updating the Library didn't help reduce the number of thumbnails. This duplication only showed up on the PLEX server. When using my Roku PLEX client there were no duplicate thumbnails showing up. I spent a couple of days trying to figure out what was going wrong by removing files and finally when I removed "X+Y" those duplicated thumbnails disappeared. I renamed the movie to "A Brilliant Young Mind" to end the problems and everything returned back to normal.
So I guess you need to be careful of what you put in that Sort Title field because it can cause some unrelated problems not necessarily having to do with sort order.