I'm not going to actually look, but I've probably featured Ulric Collette's series Genetic Portraits here before. Collette photographed family members in the same pose and then digitally stitched them together. The resemblances and differences between family members are fascinating. (via jenni leder)
Update: A similar series by Bobby Neel Adams. (via @geedix)
Update: See also these similar paintings by Daevid Anderson.