I had a similar problem with lots of files with file name ****a.nef or ****a.tiff etc. The only way I could get it sorted was a laborious search for files with filename a.nef/a.tif changing the start letter to b, c, d etc. When I found the duplicates I put them in a dedicated directory called "duplicates", my imagination know no bounds:), and then later checked to make sure I was not getting false positives and accidentally deleting the wrong files.
It is a bit of a pain in the 'Glutious Maximus' but if you persevere it can be done. However, if you have a really large number of files like me you may lose the will to live. If this happens a drink of your favorite tipple is a good cure.
Good luck.