Eliminating Duplicate pictures in Photo

Photos has no built-in tool to search for duplicates in the library. It relies on the duplicate detection on import.

To find duplicates sort the photos by the capture date. Then duplicates will appear side-by-side.

For a more sophisticated search you need third-party applications. I can recommend PowerPhotos, that will search for duplicates based on the file attributes, or Power phots, that will compare the bitmaps or histograms of the photos, to find similar photos, even if the file attributes and the file format are different.

But do not use any software, that will modify the Photos Library directly. some duplicate remover will damage your library beyond repair and you may lose photos: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support