I did some testing on this issue and found the following to be true:
any image with a date/time original prior to 2001/01/01-00:00:00 will show this symptom (i.e. duplicates not being recognized)
The colum "importHash" in table "AgLibraryFile" shows a negative time stamp for date/time original < 2001/01/01-00:00:00
(i.e. -22136634:04190019.JPG:1298237)
Timestamp in importHash is "-1" for 2000/12/31-23:59:59, and "0" for 2001/01/01-00:00:00
It looks like the negative timestamp values in importHash seem to confuse the duplicate detection process.