Photos is a bit slow to present duplicates. For a large library it may take months until all duplicates have been found. I would just wait a bit longer. The reason why it is taking so long is, that Photos seems to use the result of the scan for objects and scene types to compare the content of the photos.
There should not be any true duplicates in the library, as Photos is alerting us on Import, when we are trying to import the same photo twice, sp photos will mostly show similar photos as duplicates.
If you are in a hurry to find duplicates, you could try one the third-party duplicate detectors, that are known to be safe and do not damage the Photos Library. I am using Power Photos as a second browser for my Photos Libraries, to merge and split libraries, show the metadata in a list view, and more. It also has a duplicate detector, but you would need the full, paid version.