When importing photos into Lightroom, Lightroom is supposed to detect duplicate photo and not import the duplicates photos. How does Lightroom know that it’s a duplicate photo? Lightroom will flag a photo as a duplicate if:

  1. The original filename must be the same.
  2. The EXIF capture date and time must be the same.
  3. The raw data or the JPEG data must be the same. The preview does not have to be identical.

If you have changed the format of the photo, Lightroom will not recognize that the 2 photos are duplicate. If you have converted from raw to DNG, Lightroom will not flag the 2 photos as duplicates.

Lightroom doesn’t care which folder/directory, your photos are. Lightroom will detect the duplicates across directories/folders.