I figured it out, possibly. From time to time I take a look at my Music directory and just now found three folders with slight name variations of the band Jane's Addiction.
To resolve that issue I batch edited (in mm) the metadata for all the tracks to standardize on the name Jane's Addiction.
The live album was already tagged with Jane's Addiction, but I selected all songs by that artist and used MM to change the Artist and Album Artist fields.
As soon as I applied the change, the filenames of the Live album in mm all changed to add (2) to the filename, a full set of duplicate files were created in the directory AND the old, mistagged, files were copied into the Jane's Addiction folder, leaving the originals intact and un-deleted.
What I THINK happened was that editing the metadata in mm caused completely new files to be created, rather than updating/over-writing the originals.
What I EXPECT to happen is for mm to update the original files and for those that had an actual change (janes addiction to Jane's Addiction, for example) is for the changed files to be moved into the Jane's Addiction folder and the old folders be deleted. That's not happening:
