Combining Photo Libraries without duplicates?

Hi,

There are 2 ways you could merge your libraries, the first is to use Power Photos. You do have to pay for it, but its a very good and well respected app. The app also has a duplicate function so any duplicates can be identified.

The other option is to use iCloud Photos. By enabling it on one library, letting the upload complete and then activating iCloud Photos on the second library all new photos will be merged and most (hopefully) all duplicates will be identified and not uploaded.

However, you say that you keep a backup on the NAS, I hope this is via Time Machine. The Photos library is quite particular about the format of drive that it is saved on and a NAS drive is NOT a supported format. I'm sure it has worked fine so far but there is a very high risk that at some point in the future the library could become corrupted - there are plenty of examples on here where this has happened to people. The library should only be saved on an APFS formatted drive connected by USB.

Here is the manual page for PowerPhotos about merging libraries. https://www.fatcatsoftware.com/powerphotos/Docs/merge_libraries.html

PowerPhotos should automatically offer you to create a backup copy of the destination library, before it is merging the libraries.

Merging with PowerPhotos is working well, as long as the libraries are not syncing with iCloud Photos.

Oh, and before you undertake any merging of libraries or duplicate removal please take backups of both libraries either stored on your Mac or a USB formatted drive as mentioned above. You can also use the format MacOS extended journaled - with the ignore ownership flag checked.

Bear in mind that to use the icloud merge method, you will need enough iCloud space (at least for the time it takes the merge to complete) to store the entire merged libraries. You haven't stated what "massive" means.