Yes, I searched for this problem, and found a few posts and answers, but none are the same situation.
I have an external drive in my library. All folders have been imported in LR. I've been working on and editing a few hundred images over the past few weeks. When I unplug the drive, naturally the library has all the question marks beside the folders. I plug the drive in...they slowly repopulate like normal. All of a sudden, I plug the drive in today, half the folders show normal, but a bunch of others remain with question marks. I go to "locate" the folder, but nothing happens. When I go to locate one particular file within the folder, LR gives me an error saying the "photo is associated with another photo in the library". But it is the only photo. I haven't moved any folders or files around. In fact, some of the folders on the same drive, LR recognizes, others within the same folder, it doesn't. Even stranger, some photos within a folder are recognized, others are not.
I tried removing one of the folders from my library. Then imported a few photos within that folder (from Photomechanic) and again, get the very same error messages.
This means the photo has been imported more than once (probably accidentally), and that's why you get this error message.
You need to find both copies of one of these photos in Lightroom. Follow all of these four steps in order:
1. In the Lightroom Classic Library Module, on the left, under Catalog, click on All Photographs 2. Turn off all filters (Ctrl-L once or twice) 3. Turn off all stacking (Photo->Stacking->Expand All Stacks) 4. Search for at least one of these photos by file name using the Lightroom Classic Filter Bar
Once you find the two copies, then we can advise on the next steps.
Just a manually idea to verify what DJ-Paige said, you could close LR, and then using the Finder (Explorer) look at the folders and count the number of files in there - on a mac, the bar at the bottom of the windows tells you the number of items in a folder.
If the number of images in the folder is exactly the same as the number of images in the folder in LR catalog, then you are probably trying to inadvertently reimport the same photos.
Lightroom actually uses more than just the file name to search for duplicates.
Also VIrtual copies in LR can complicate the issue.