iPhotos

Photos for Mac does not have a tool to search for duplicate photos. It is searching for possible duplicates, when you import photos, so there should be no duplicates, unless you are importing edited versions of the same photo.

I am using the following duplicate detectors, when I suspect duplicate photos, but both are not free:

• PowerPhotos: It can find duplicates based on the file attributes and the capture date. But it cannot find similar photos with different dates or filenames.
• Photo Sweeper: It can find duplicates, if the photos have been renamed, downsized , or adjustments applied.

Some duplicate finders can damage the library. Do not use an application, that is deleting the duplicates directly from the library without using the Photos interface. Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

You can try and install the App "Cleaner One for iOS", check out the link provided below.

https://apps.apple.com/us/app/cleaner-one/id1156773866

Search photos see this article https://support.apple.com/en-in/HT207368

Using third party apps / softwares to find duplicate photos can damage photo library https://support.apple.com/en-in/HT204968

You can manually delete duplicate photos see the thread https://discussions.apple.com/thread/250983357

Just to confirm, is this *Photos* or iPhoto. You posted in the Photos forum.