I've just released Mp3tag for Mac v1.8.10 with the following changes:
New
- Added support for removing only duplicate fields that have the same content at action Remove duplicate fields.
Changed, Improved & Fixed
- Changes that were made in individual files via navigation in Extended Tags were written to a different file in rare cases.
It's a free update! You can update the Mac App Store version via the Update tab in the App Store app and the standalone version from within the app via File → Check for Updates .