Photo
iPhotos froze up while I was moving photos to a new album. It asked me if I wanted to import a photo that was a duplicate, when I attempted to answer telling it not to duplicate my iPhoto program froze up. I am not able to close iPhoto.
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What app are you using: iPhoto or Photos?
In either case go to the menu and select Force Quit. Select Photos or iPhoto and force quit it. Then relaunch the app and continue.