Problem with Plex photo library

I created a photo library on my brother’s PC using the same photos and folder structure as on my PC. The photos look great on my PC, but on my brother’s PC, there are numerous duplicate photos. If I try to delete any of these duplicates, Plex just scans them again and re-adds them. I tried deleting the Photo library, and re-adding it, but got the same results.

How can I completely delete the Photo library, and any cached files?

-Thanks