Works quickly, and seems solid

Having my photos library begin with the purchase of my first digital camera in 2003, and morph across every version of iPhoto until I pretty much broke iPhoto with my library size, then migrating to Aperture that was far more robust, until support for Aperture died, and finally into Photos, it can be easily said that my library is a mess.

One weird quirk was that when moving from iPhoto to Aperture, the albums often duplicated images, leading to a shed-load of duplicates. I had manually fixed a lot of this, but gave up the Sisyphean task as too tedius.

Photos Duplicate Cleaner came in and quickly found the pile of ducplicates, and made it trivial to cleanse them from my library. And it is free.

Gander666 about Photos Duplicate Cleaner, v1.7