A congenital defect where parts or all of the face is duplicated on the head. Unlike conjoined twinning, diprosopus is not the result of the fusion or incomplete separation of two embryos. Instead, it is caused by excess amounts of the sonic hedgehog homolog (SHH) protein. The SHH protein and its corresponding gene plays an important role in craniofacial patterning during embryonic development. In excess amounts, the SHH protein can lead to the widening and duplication of facial features.

Few animals with diprosopus survive because of brain or internal organ abnormalities that often occur with the congenital disorder. Most infants with diprosopus are stillborn. Some of the most famous include Ditto the pig, Janus cats Frank and Louie, and Indian baby girl Lali Singh.