Duplicate photos not being automatically identified in Photos for Mac

I upgraded to macOS Ventura (13.2.1) in January and was pleased to find the Duplicates facility in Photos (v. 8.0). Having 25,000 photos in my Photos Library on my iMac 24" M1, I was soon aware that it could take a while to undertake the search for duplicates. Eventually, a total of 42 were identified and I used the merge facility to resolve all these except for two (very similar photos but not duplicates).

However, I also later identified a number that the system had not picked up, so I have spent the last few days manually identifying a further 288 duplicates so far. These are waiting for me to find the time to process them.

In researching a possible reason for the problem, I found one website (https://macpaw.com/how-to/duplicate-photo-finder-for-mac ) which included the statement:

“When you import your pictures, the Photos app automatically detects duplicates, but it won’t help you with the ones already in your library. For those images already in your photo library, you’ll have to take a more manual approach.”

I can find no reference to verify this statement anywhere, particularly on the Apple Photos web page. I query whether the statement can be true since all the duplicates which had been automatically identified were already in the Photos Library having been imported some time ago. These were a combination of those taken digitally and those scanned in.

Could anyone shed some light on the statement above and/or suggest why the 288 duplicates found manually had not been picked up automatically?