Photos - iPhone shows "New Items" but photos are already in Photoslibrary

You may also seeing old photos as new, if iCloud Photos has become enabled on your iPhone. the download from iCloud is turning the photos into new files. Also, when you are taking photos in the heic format, but are converting them to jpeg on import.

Is it possible that you used a different method to import that photo back in 2002? Over the years, there have been reports that different import methods can create slight differences in how photos are imported that can interfere with the detection of duplicates on subsequent import attempts. There was an import method called My Photo Stream that did not always mix well with cable-based imports, as I recall.

That's another example of how mixing import/sync mechanisms can confuse the situation.

Indeed. For importing we strictly should follow "pick one horse and ride it", Frank Caggiano wrote once here. 😀