Photos claims photo already imported but search finds nothing

I was trying to import about 15 photos from a folder on my macbook pro to my Photos library which is on an external drive. Photos showed that several of the pics had already been imported and found only 6 new. It imported the 6 and then I tried to find the ones it said had already been imported and couldn't find them.

I checked the Exif data to make sure there was nothing messed up with the dates and there isn't. Still, decided not to trust myself and did a search of the file names and Photos shows "No results". For each of those photos, I tried to import them individually and I get "Would you like to import the following duplicate item" and I get the two little thumbnails of the photo that I'm trying to import and the photo that supposedly already exists in the library but that Photos can't find when I do a search!

Can anyone hazard a guess as to a) what's going on, b) whether or not the photos are indeed in the library, and c) if they are not, how I can convince Photos to import them?

Many, many thanks.

NB: I have no hidden photos.