The built in Photos duplicate feature is the best choice for most people: it’s not just generic file-level dedupe but smart enough to do things like take three

The built in Photos duplicate feature is the best choice for most people: it’s not just generic file-level dedupe but smart enough to do things like take three versions of the same photo and pick the highest-quality one, which is great if you ever had something like a RAW/TIFF+JPEG workflow or mixed full res and thumbnails.