I have a rather large Photos library (> 100GB).
I am in the process of tidying up a few USB hard drives containing, among others, a bunch of photos.
How to check which photos on these drives are already in Photos and can be safely deleted?
If you don’t use iCloud to sync your library, it’s super easy to archive off a copy of your Photos database (in addition to your normal backup) and then just run the import and see how bad or good a job Photos does at not importing duplicates.
Many situations, you’ll have a great import and be done.
Before I cared about the faces metadata, I did also validate you can detach the library from iCloud and then do an import like above, but now that I have my face data set up amongst my various iCloud synced devices, I am loath touch anything in iCloud relating to large changes in photos. (As that implementation is too opaque, fragile and lacks tools to fix in 2019 IMO)
So, for me, I would import the photos into a second library and then use the smart merge function or a third party tool Power Photos to avoid duplicates and keep the peace in my ~180 GB cloud synced library of photos.
This software is so powerful and worth it if you spend maybe a dozen hours a year cleaning duplicates or just love having a tidy library. There’s no harm in letting Photos try to import and detect duplicates if your time budget has tons of time to clean up and your money budget is tight, but I recommend a tool that’s designed to do a better job than Photos.
This tool will let you split your large library as well easily, so it has tons of features in addition to just merging.