Dealing with Duplicates
When importing images from a CD or a folder on your computer, or even from a camera where you left the original images on the card after a prior import, you run the risk of importing photographs you’ve already imported. Luckily, Photos has a built-in duplicate detector. If you try importing a photograph that you have imported before, the Duplicate Photo warning appears.
The Duplicate Photo warning gives you the option of reimporting the photograph, which adds another copy of it to your library, or not importing it, which causes Photos to skip that single photo and move on to the next. If you check the Apply to All Duplicates check box, Photos either imports all duplicates or doesn’t import all duplicates in the current import session, depending on whether you click Import or Don’t Import.