remove duplicate photos?

Photos does not have a tool to help us to weed out duplicates in our library. Photos is scanning for duplicates when we are importing photos, so we do not accidentally import the same image file twice. But if we import the duplicates nevertheless, we are on our own. Ironically, Photos 5 or later is using a lot of processing power to search for duplicates. But it does not present us a list of duplicates for removal. It is using the information about duplicates and similar photos to hide the clutter from the "Days" view, see: About the Duplicate Detection in Photos 5 - Apple Community

If you have a lot of duplicates in your library, try to find out what has been causing the duplicates. Knowing the source of the duplicates might help to search for duplicates with smart albums based on the filename extension.

You could also use a third-party program to search for duplicates and collect them in an album for you. But be careful. which program you use. Some app can destroy the library and you may lose the photos.

Only duplicate finders that do not modify your library directly are safe to use, for example Power Photos. But you need the full, paid version to remove duplicates.