Why doesn't "Don't Import suspected Duplicates" work?

I import files from my SD Card and do not edit them.  Put the card back in the camera and take more photos.  Import the files to LR Classic again and the photos from the previous import show on the card as new images and LR wants to import them again.  This used to work and uncheck the previously imported files.

What has changed?

Using macOS and latest version of LR.

Thanks,

Dave