When I updated my iPhone 7 to 12.1, all the pictures that were on my phone merged with my wife's photos as well. So I went from 11,000 photos to over 35,000. There must be an easy fix to this, but I'm not very knowledgeable about this stuff and am worried I'll end up deleting all of them. Suggestions on how I should proceed?
If you don't have a backup, you will probably need to duplicate the photo library and then categorize / delete the files by hand.
Have you exhausted Apple support to locate the various backups before they might expire or get overwritten?
Also consider checking in iCloud photo library to see if that's the cause. Sharing an AppleID between people can do this exact thing which is why having your own iCloud is often a very big time saver.