How to merge Photos Library files and determine content overlap on an Apple M2 MacAir?

When you import photos from one Photos Library into another, Photos will check for duplicates and show you the items that are only in the imported library and missing from the destination library in a separate list as new items. Import only these new items, see: Import from another photo library on Mac – Apple Support

I suggest the following:

• First look into the two small libraries, if the photos in them are relevant at all. Given the very small size, there can only be a few photos in them. The names suggest that one is a copy of the other, but the libraries may have been changed after you created the copy.
• If the photos are worth saving then merge first the two small libraries into one library by importing the new photos from the copy into the older version. The size should not change much.
• Then import the joint small library into your current library, again only the photos listed as "New".

All the things léonie said, and

Generally when a Macs makes a copy of "Photos Library.photoslibrary," it would be "Photos Library copy.photoslibrary" like one that you have. "Photos Library 2.photoslibrary" might have been made by some other app.

You import pictures from a different Library by using File>Import and selecting the other Library. It will import only pictures and their metadata, but this will not preserve the album and folder structure. If that's important, then you can add the album name as a keyword to all the pictures in each album in that Library so you can collect them together later in the receiving Library.

léonie suggests merging the two smaller Libraries first, because merging with a larger Library will take a lot more time, since it has to compare with way more pictures for duplicates.

Let us know how it goes…

Perfect! Thanks for letting us know.

There could not have been many pictures in the two libraries anyway, as they have been so small.

I love it when it turns out so easy!