Photos in albums are no duplicates. Albums are just referencing the photos in the library that you are seeing in AllPhotos. If you delete a photo fromAll Photos, it will vanish from all albums tat are using the photo. Think of albums as a catalog with index card. The one copy of the photo is stored in the library, and the albums are just another view of the photos, a list of photos pointing to the one copy in the library.
If you want a view, where you can find all photos that you have not yet added to an album, create a smart album.
- File > New Smart album
- Set the rule to "album is not Any"
This album will show you all photos that you have not sorted into an album.
Be careful with duplicate detectors. Some of them are very aggressive and can damage your Photos Library badly. I would not use any duplicate detector, that is automatically removing duplicates without collecting them in album, where you can delete them yourself. Apple is warning against such applications: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support
But your photos are no duplicates, There is no way in Photos for Mac to have a photo in an album but not in All Photos.
