Lightroom claims photo already in catalogue, but can't be accessed

Response deleted by the author.

Synchronizing the folder just imports the photo a second time, without your edits and user-provided metadata; or to be blunt, you have made the problem worse now, with the photo imported twice in Lightroom Classic's catalog but each one has different edits and different user-provided metadata.

It may seem sensible to synchronise the folder tree where the images now are - or, which amounts to the same thing, to import them again from that  folder tree. It may even, on the face of it, seem to have achieved what you thought was required. But that's not what has in fact happened.

Analogy: you are in charge of a company's employee records. You discover that the address details you have got on record, for each employee, have (some, or all) become wrong because of a recent external change. Say: the area postal code has changed. So when you try sending mail (to what it currently says in the employee record, still showing the old postal code), that mail won't get delivered any more. But otherwise these employment records are fine - salary, next of kin, training history etc.

You now have two alternative courses of action. Which one makes more sense?

• Set up a brand new empty file for each employee, in a new file cabinet. Enter into this their address details including the new area postal code. Their salary etc would all need to be found out again, and typed into this new file, before it will be as useful as the previous paperwork... which you have now decided to ignore - even though the only thing ever wrong with it, was the wrong postal code. At some later point you may destroy that previous paperwork (on the strength of its non-working mailing address) - but until then there remains ongoing confusion between these two apparently separate "employees".
• You find out how the postal code needs to be changed for all the addresses you already have got, in the established files that you already have got, which otherwise contain full up-to-date employee details.