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When importing images into Lightroom, you have the option of "All Photos" or "New Photos". When picking the latter, LR seems to eliminate photos that it already has in the catalog. Once in a while, though, I've noticed that it is fooled and imports duplicates.
What is the method that LR uses for determining already-imported photos?
This is answered right in the Adobe Photoshop Lightroom documentation on importing files:
Lightroom determines a photo is a duplicate of another file in the catalog if it has the same, original filename; the same Exif capture date and time; and the same file size.
