Duplicate Photo Libraries

It will depend on your system version. In macOS 12 Monterey or later you can import the photos from one library directly into another library. Photos will check for duplicates and skip the duplicates when importing, see this page in the Photos User Guide: Import photos from another library in Photos on Mac – Apple Support (UK)

If you have a lot of organizational effort in both libraries, i.e. albums, titles, captions, keywords, etc., you might consider using the paid version of PowerPhotos.

Here's the differences between Photos and PowerPhotos regarding merging libraries:

PowerPhotos: Photos:

Albums ----------- = (does not provide this)

Smart Albums (as regular albums) -----------

Folders -----------

Captions Captions

Titles Titles

Original images Original images

Edited images Edited images

--------------- Locations

Favorites Favorites

Can detect and exclude Duplicates detect after import

duplicates or not during import

Neither method included projects.

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library.