Starting in iOS 16 and macOS Ventura, Apple added a feature to their Photos app that can detect and eliminate duplicate photos. The two function pretty similarly overall, but Photos’ system works differently from PowerPhotos in a number of ways, both with their advantages.
Photos system works automatically in the background, and will display a “Duplicates” album containing what it has found when it’s finished searching. It doesn’t give any progress information or let you control how or when it searches though, and Photos’ duplicate detection only operates on a single library at a time.
You can browse through the duplicates that it’s found and choose to merge one or more duplicate pairs together, which will take various data from both copies of the photos such as metadata and editing information and combine them into a single photo in the database. The exact way this works isn’t documented by Apple and doesn’t offer much control, but does a good job of trying to retain as much information as it can.
With PowerPhotos, you perform an explicit duplicate search, and can specify one or more libraries to search through. PowerPhotos does a similar visual duplicate search to Photos, but offers additional options that let you control whether other aspects of the photos need to match too, including things like filenames, dates, and photo size.
PowerPhotos doesn’t merge photos info like Photos does, but instead gives you control over choosing which photo to keep from each group of duplicates that it finds, either using its built-in rule system for automatically choosing keepers based on a number of criteria, or by manually selecting keepers yourself. In addition to simply deleting the extra duplicates, PowerPhotos will let you collect the extra duplicates into an album instead if you’d like to review them further before deleting them.
Which of the two will find more duplicates depends on the individual library - sometimes Photos will find more, and sometimes PowerPhotos will find more. Feel free to use both of them on your library to get rid of all the duplicates both of them can find. If you prefer Photos method of combining photos then try running it first, then PowerPhotos, or if you need PowerPhotos greater control, you can run it first instead.