A magical camera with the power to create a literally negative replica of a person whose photograph is taken with it is called the duplication camera. Once the photo is developed, the evil replica will be born. To destroy the evil replica, the only option is to destroy the developed photo. It may not work on vampires because their reflection would not transfer into the camera.
In Double Negative, Benny found it in his grandmother's basement and sold it to the yearbook committee for 10 bucks. The yearbook editor Hannah Price accidentally created an evil twin when she took a photo of herself with it. The evil twin terrorized the school, prompted Benny to take a photo with his back facing the camera for revenge, creating an evil twin of him, too.
Evil Hannah and Evil Benny devised a plan to create an entire group of evil replicas by taking a class photo. Before they had the chance to develop it, however, Ethan destroyed it with hydrochloric acid (a corrosive acid), taking Hannah's evil twin with it. However, Evil Benny stole his own photo beforehand and emerged to take a photo of Ethan, then locked him up in the dark room.
The real Benny managed to steal his photo and flush it down the toilet, ruining the picture and thus destroying his evil twin and the rest of them.