To be honest, this is the first time I have encountered this, but I don't understand why.
When you import from an SD card that has both old (previously imported) and new photos, LR recognizes those that have already been imported and does not "check" them in the import dialog. Recently, this has not happened - LR checks for import all the photos on the SD card, including those already imported. Why would this be happening? I have not changed any file names.