What does it mean when there’s a little blue box in the top left-hand corner of a movie with a number in it?
It means Plex has found several files on disk, which it thinks are the same movie/episode/video.
Inspect the Plex media info of the item to see the file names and eventually resolve this.
some more info
How to find the discrepancy between number of files on disk and the number of movies in the Plex movie library
