In Double Negative, Benny's Grandma's Camera, which Benny had sold, unaware what it was, fell into the hands of the yearbook editor Hannah. However the Camera was cursed, and when the film was developed, it created a Doppleganger of whoever the picture was of, creating one of Hannah, and later Benny.
Their Doppelgangers plotted to make an army of Dopplegangers by photographing everyone, but Ethan managed to destroy the negatives before they were created, and Benny destroyed the picture of him, thus destroying the Doppleganger.
In Independence Daze, Sarah's pocket dimension, had a Doppleganger of her, representing her worst fear, of her becoming a monster. It was destroyed when Benny and Ethan lured it to them, and Sarah staked it.
They possess both the appearance, and all the skills (including any supernatural powers) of the person they are copying, however they are much darker, and represent the most negative sides of their personality, and they also often hold contempt for the orginals.