If you import photos taken before 1st of January 2001 00:00:00, duplicates will not be detected even if you select the "Ignore duplicates" option on import. The files will be imported twice into the same location with a "-2" postfix added to their name.
Technical details (to help developers):
To detect duplicates, Lightroom uses a so called import hash (AgLibraryFile.importHash in the database) that has the following form:
<capture_time> - interval in seconds between 1st Jan, 2001 00:00:00 and capture time recorded in Exif,
<original_filename> - name of source file before import,
<file_size> - file size in bytes.
The problem here is the <capture_time> part, since it gets negative for files older than 2001, since 1st Jan 2001 is sort of considered the beginning of time (called "epoch" in computing). Lightroom seems to get confused on negative timestamps.
I will report this bug. Thanks!