Face duplication on the head. Due to the sonic hedgehog homolog (protein) that plays an important role in signaling craniofacial patterning during embryonic development e.g. governs the width of facial features. In excess it leads to widening of facial features and to duplication of facial structures. Insufficient amounts of that protein lead to opposite conditions such as cyclopia where facial features are insufficiently developed. Most human infants with diprosopus are stillborn.
Information adapted from Wikipedia.