photos - original library + other folders of photos outside of library that may have already been imported???

Hi - I've recently ported my Photos library from an older iMac to a newer MacBook. I have the same library currently appearing on my iPhone, Watch, iPad, and everywhere else I'm logged in with my Apple ID. Additionally, I have set up a shared library with my wife --> where all the photos from the library exist. So far this process is working.

The question I have is I also have many other folders outside (external drives, etc.) of my Photos library that contain thousands of photos. I suspect (and have confirmed) that some of those photos were previously imported into the Photos library, but am not sure if all of them are there. I can't possibly review each photo outside of the library and then compare it with photos inside the library as that would take hours and hours. So...what do I do? Should I import every photo that lives outside of the library into the Photos library and then hope that the system (Photos/iCloud) recognizes them as duplicates and work through the process? Or is there some other alternative. Thank you!