Error message "the file [x] is associated with another photo in the catalog"

I have several photos in a Lightroom folder. The folder icon has a ? mark on it. Some of the photos in the folder have the ! icon indicating that the "Photo is missing". I right click to try to Locate the file. LR navigates to where it thinks the file was last. The file IS THERE, but when I click on it to relink it, I get the error message "error message "the file [x] is associated with another photo in the catalog".

I've seen this problem elsewhere on this forum, but my situation does not match exactly, so I'm asking for help here. So let me continue with more information and what I've tried.

If I right click on one of the images in the folder where the "photo is missing" and select "Show in Finder" it goes right to the photo in the same location as doing the "Locate" function. The file is there and intact.

If I go to "All Photographs" in the Catalog module, and search for one of the missing files, it returns only one entry, the same entry with the missing file. (I expanded all stacks, and cleared all filters).

The missing photo I'm working with (and the others) are part of a collection that is synced to Lightroom (so I can see the collection on my iPhone running the LR app.

So at this time I am stuck. I can find no way of relinking the only file I seem to have and clearing the ! icon.

Any help would be greatly appreciated.

Thanks.