Something remarkable:
I'd created a .m3u playlist for October 29 with a dozen songs and, among those dozen songs, directions to another .m3u playlist file nested inside the October 29 playlist file. The october 29 file was one that was duplicated.

This doesn't seem to be the same issue with the October 27 .m3u file.

Maybe it has something to do with the file names, like the fact that whatever recognizes file names in the file system can't, directally, change a file name from a lowercase to a capitolized letter. ...