You do not have to move/transfer all your movies as you can filter the list using the wildcard character"*" i.e. dir *.avi /b > avimovies.txt, dir *.mp4 /b > mp4movies.txt, dir *.mkv /b > mkvmovies.txt, etc. You will have individual txt file listing all avi, mp4, mkv, etc movie files.
Or you can use the "/o" switch and the "sortorder" of the dir command which will sort the list. Hence, if you run "dir /a:-b /b /o:e > myfiles.txt" (w/o the quotes) will create a text file (myfiles.txt in my example) with all the filenames sorted out by extension. You can then highlight, in Notepad, the group of avis, mp4s, mkvs, etc. and paste them in Excel. In Excel, you can sort the titles and see which movies are doubled (same file name but different extension)
The dir command's "/a:-b" switch is used so that directories are not listed, the "/b" switch uses the bare format (only filenames and extensions are listed) and the "/o:e" switch sorts the list by extension. In dos prompt you can type dir /? to see other options.
If you are not up to or not familiar with running dos commands, you can try moreeg's suggested program above.