I have photos on a flash drive, which is where I would like to keep the files. When I try to import the photos, I do not have the option to add the files, only to copy. Copy gives me duplicate files. Do I have any way of adding as opposed to copying?
Lightroom is protected against adding images from a removable drive, because it can't see the difference between a flash thumbdrive and a memory card. If people could use 'Add' with the memory card of their camera, they would lose the images if they did not realise what they had done and formatted the card to use it in the camera again.
If you really want to store the images on a flash drive, then use copy and afterwards manually delete the originals in order not to have duplicates.