I rsync my main library regularly to my cabin's NAS, both as a backup and so it can play the library locally rather than consume bandwidth. Over time this has led to a number of movies having multiple video files as I upgrade file resolution/quality. This linux shell command will list all directories that have 2+ video files inside:
find . -type d -exec sh -c 'set -- "$1"/*;X=0; for args; do [ -f "$args" ] && [ '.mp4' = ".${args##*.}" -o '.m4v' = ".${args##*.}" ] && X=$((X+1)) ;done; [ "$X" -gt 1 ]' _ {} \; -print
The [ '.mp4' = ".${args##*.}" -o '.m4v' = ".${args##*.}" ] segment can be modified to include other file extensions by inserting -o '.ext' = ".${args##*.}" for each one inside the two square brackets.