Eradicate detected duplicates on disk

Hi, I'm trying to find a way to help a friend with a Lightroom Classic riddle: he has a beautiful folder structure on disk, but at some point in the past, when weeding out, accidentally chose "remove from Lightroom" instead of "Delete from disk".

Unfortunately, this wasn't just for rejects, but also for (plenty of) duplicates.

The entire folder structure contains some 50000 photos.  We used the Import dialog and chose a "move" operation for the photos that were rejects (not duplicates), to get them OUT of his folder structure, and then just delete them for good. That was the easy part! 😄

Now the question is: how the heck do we do the same for the duplicates? Using "Synchronize Folder", the dialog window tells us that there are ~5000 new photos. When we use an Import with the Duplicate check ON, there are zero new photos. In other words, all 5000 "new" photos are indeed duplicates.

When I turn the duplicate check in the Import dialog off, LRC sees all 50k photos in that folder structure... and we don't want to mess that folder structure up, of course. How do I identify the duplicates efficiently, without having to import them, making a mess in the existing folders?

Any ideas or help would be appreciated, thanks.