Thus I just ran my program on the Masters directory.
One risk would be losing edits to images, but I was OK with this.
We also had different images existing with the same name, due to the counter on the camera looping around.
This was OK since i Photo stores images from different imports in different directories, but it meant that we couldn't just copy all the files into one directory and use the names to de-duplicate them.
i Photo has a menu item for finding duplicates, but I was not been able to make it work; it would just hang trying to find duplicates in one of the smaller libraries.
And even if I could get that to work I didn't have the space to copy all the images into one giant library and let i Photo sort it out. One problem was that the same picture would exist with different file names, probably due to being imported from devices into multiple different i Photo libraries or to being exported & then imported again in the aforementioned failed consolidation attempt.
Instead of being limited to putting all your photos in a single library, Power Photos can work with multiple Photos libraries, giving you many more options for how to organize your ever growing photo collection.This explains how I consolidated & de-duped about 45,000 pictures and movies from four i Photo libraries on two different machines and various directories full of images on USB drives into one 33,000 file library.