If you use the Windows 10 Photos application to import photos from an SD card or camera, note the following to avoid creating duplicate files:

• Selecting a folder that was imported previously will cause duplicates of photos. The duplicates will be placed in the same folder but renamed with a number at the end, e.g. IMG_1234 (1).jpg
• A folder will not be automatically selected if Photos recognizes it has previously imported those photos. DO NOT MANUALLY SELECT ANY folders.
  If nothing is selected to import by Photos then you have already imported those photos!
• You can also avoid duplicates by deleting the photos from the card/camera after importing.
• If you delete, rename or move a photo, Photos will not import it again automatically.
• If you have accidently re-imported a lot of photos you can use a duplicate file remover tool to find and delete the doubles.

Why does Microsoft do such a bad job of a simple task…?!