One method that I've found to work with both broken thumbnails and duplicate thumbnails is to move the photo files somewhere else, then move them back. More precisely:

1. using File Manager, move the affected photo files to a different folder.

2. start the Photos application. Wait for the thumbnails to appear. Close the photos application.

3. move the files back to their original application.

4. start the Photos application. Wait for the thumbnails to appear.

If all your photos are affected and they are all in the same folder (e.g. DCIM), select the folder and move it somewhere else on the same drive (internal storage or SD card). This avoids physically copying the files to a different drive and is quick. The longest wait will be to recreate the thumbnails in step 4.

I guess one might even make a script to automatize the above procedure.