Merge Photos libraries help

[motulist]

For emergency reasons I had to download all the photos from my iphone onto my spare mac.

So now my spare mac has a Photos library that contains all the new pictures from my iphone, and it also contains thousands of pictures that already exist in main mac's Photos library.

Now I need to move the new pictures sitting on my spare mac and transfer them onto my main mac. I need it to only copy over the new photos, without duplicating the pictures that already exist on my main mac.
---> How do I do this?

Main mac receiving the photos is on 10.14.4
Spare mac sending the photos is on 10.13.6
I don't have iCloud space

 

[Slartibart]

Just import the Photos library from “spare Mac” into “main Mac”. Then use e.g. Photos Duplicate Cleaner.