https://4allapps.com/powerphotos-1-7-7-for-mac-photos-libraries-find-duplicates-and-more/
PowerPhotos
PowerPhotos allows you to break up your Photos.app images among multiple Photos libraries, rather than having to store all of them in one giant library. You can easily switch between libraries, or just browse the photos in your libraries directly from PowerPhotos, without having to open each library in Photos. PowerPhotos also supports searching across all your libraries at once to help track down a particular photo. If you use iCloud Photo Library to sync photos with your other devices, splitting up your library can help save on iCloud storage costs. Use a single library to hold a smaller collection that you sync with iCloud, while keeping the bulk of your photos in separate libraries so they don’t take up space on iCloud. PowerPhotos can analyze your libraries for duplicate photos, showing them to you side-by-side and letting you get rid of extra copies of photos that you no longer need.
Create and manage multiple libraries
Instead of being limited to putting all your photos in a single library, PowerPhotos can work with multiple Photos libraries, giving you many more options for how to organize your ever growing photo collection.
Splitting your photos between multiple libraries helps Photos open more quickly, lets you archive older photos, store some of your collection on an external hard drive, and reduce storage space used by iCloud Photo Library.
Copy photos and their metadata
You can split your library by copying albums and photos with a simple drag and drop. PowerPhotos will take care of copying the photos and retaining their metadata, including keywords, descriptions, titles, dates, and favorite status.
Merge Photos libraries
Got a whole bunch of libraries you want to consolidate into one? PowerPhotos lets you merge your libraries together while weeding out duplicate photos in the process. You will be shown a preview of what your merged library will look like before any modifications are done, to ensure your merged library looks how you want.
Find duplicate photos
It’s easy to have multiple copies of the same photo creep into your photo collection over time. Use PowerPhotos to find duplicate photos in one or more libraries, view them side by side, and separate them into albums where you can easily delete them from your library.
Browse and search
Use PowerPhotos’ image browser to quickly view your photos without having to open Photos itself. View detailed photo information using List View, and search for photos across all your libraries at once.
Migrate your iPhoto/Aperture libraries
If you have a lot of existing iPhoto or Aperture libraries that you want to migrate over to Photos, PowerPhotos will help automate your migrations so you don’t have to spend time babysitting this lengthy process.
Requires macOS 10.14+
