So I set my camera up exactly as I shoot houses (U1 saved settings) and then changed the second card slot to be backup.
I shot three sets of bracketed exposures…total of 9 images.
I hooked the camera up to the computer, and just before importing, went to Windows Explorer (like Finder on the Mac) and inspected the two memory cards. Each had 9 photos on them. That confirms that backup actually works as expected.
Next step…back in Lightroom, clicked on Import. Below is my default setting with the checkbox on Don’t Import Suspected Duplicates. But what Lightroom displayed previews for importing wasn’t 9 unique images, but 10 images (9 plus a duplicate)…I show just the unique and duplicate images below. Important Note—if I UNCHECK that duplicates box, the Lightroom previews 18 images for import—all images on both cards. When I re-check the box, the previews revert back to the same 10 images (9 unique on card 1 plus the duplicate from card 2).
After importing, I compare the metadata for the two images, and they are identical. The Date Time Original, Date Time Digitized, and Date Time are all identical. Why Lightroom felt THIS image wasn’t a duplicate but recognized the other 8 images on card 2 were duplicates is still a mystery to me. But I’m satisfied that 1) my camera works, and 2) the backup to card 2 option works as expected.
But the mystery remains…why does Lightroom think one of the backed up images is unique and not a duplicate??