You can have duplicate images on your computer for mostly three reasons:
#1. You transferred a set of pictures from the digital camera to your computer but did not erase them from the camera. Thus when you re-connected the camera, they got copied again to your hard drive but in a different location.
If you’ve got an over-exposed digital photograph that you like, don’t overlook it as a lost photo too quickly. Using layers and blending modes in Photoshop lets you easily fix your over-exposed photos and take them from “not-so-great” to “keepers”.