How can I delete duplicate photos?

I use Cisdem Duplicate Finder to get rid of the duplicates in Photos. I use the app to scan my Photos library. When it's done, the results window will display the found duplicate photos in groups for me to preview and view. The app automatically and smartly selects duplicates for me to delete with a click. If I want, I can select duplicates for deletion by hand. Also, there are selection rules (such as Select Newest) to help me mass select duplicates. When I click the Delete button, the selected files will be deleted and go to Recently Deleted in Photos app. Here are the detailed steps.

Your "Detailed Steps" links is pointing to the version for Mac, but the question is about Photos on an iPhone.

The safest way to deal with duplicates is to find out, why they are occurring in the first place and to avoid mistakes when syncing. Usually it is a problem of making mistakes when syncing photos between devices. It is important to use only one method of syncing and not to mix the manual import, iCloud Photos, My Photo Stream, import from shared albums or social platforms. And to avoid the conversion to a different format when syncing between devices. Syncing errors can create apparent duplicates by creating versions in a different size or file format.

How have you been syncing? Do you know, why you got the duplicates? Then use this information to search for the duplicates systematically.