Duplicate Photos Fixer Pro is a handy, inexpensive ($0.99) app for macOS (10.7 or higher) that can remove duplicate photos and videos from Photos or iPhoto (9.0 or later). When it works, that is.
To use it you'll have to have Photos or iPhoto running in the background. You'll also have to turn off Photo Stream and Photo Sharing (in the iPhone Preferences menu) if they're running.
Once you've done this, launch and point it to your Photos or iPhoto library. It scans your library for duplicates. When it finds duplicates, it moves them to the trash. Duplicate Photos Fixer Pro can also clean the trash to recover disk space.
However, it took a couple of attempts for me to get the utility to work. Twice it told me that Photos was "unresponsive" and couldn't scan the photo library. The third time was the charm and it ran and worked well.
Apple World Today Rating (out of 5 stars): ★★★