Facial duplication is also known as diprosopus. The word "diprosopus" comes from the Greek "di-," two and "prosopon," face and means "two-faced".
Two-faced conjoined twins have almost complete fusion of their bodies with one set of limbs with part or all of the face duplicated. The condition usually results in stillbirth, but not always as seen in this recent news report of this little girl born March 11, 2008.
Dr. Kotrikova (1st reference) writes that the "first description of a double face was mentioned in the chapter The Miracles of the Far East from Jacob van Maerlant's manuscript The History of Troyes dated approximately 1270. In the sixteenth century, Jacob Rueff, a barber surgeon, published in his Manual for Midwives woodcarvings of infants with lesser degrees of twinning, including diprosopus."
However, a New York Times article by William Honan published in August 2000 states that the ancient two-faced Mexican figurines known as the "pretty ladies of Tlatilco" are now thought to represent diprosopus. It is believed these early Mesoamerican figurines may, in fact, be "the oldest scientifically medical images in world history." Sculpted over a period of 500 years beginning in 1200 BC, these little statues show diprosopus with two faces side by side. This degree of accuracy in documenting the anatomical and pathological features of a human head does not appear for another two millennia until the 16th-century studies of Hans Baldung Grien and Andreas Vesalius who did anatomical drawings based on dissected cadavers.
Children with craniofacial abnormalities such as diprosopus are characterized by a single body and a variety of duplications of craniofacial structures. These children may have four separate eyes (diprosopus tetraophthalmus) or fusion of the medial eyes. Two, three, or four ears may be present. Some have only partial or complete doubling of the nose (diprosopus dirrhinus).
The prevalence of conjoined twins (Siamese twins) is reported to be between one in 2800 and one in 200,000 deliveries. The prevalence of diprosopus is reported to be one in 15,000,000.
Conjoined twins are classified according to their symmetry, site of fusion, and degree of duplication. The most frequent type of conjoined twins is thoracopagus (32.7 percent) and the rarest type is diprosopus (0.4 percent).
Cardiac malformations (ventral septal defect, transposition of major arteries) represent the most common congenital abnormalities. The esophagus, stomach, and intestines may be duplicated. Where there are more severe associated anomalies, the prognosis is poor.
Hat tip to Dr Kevin for the news article link.
Operative Correction and Follow-Up of Craniofacial Duplication; Plastic & Reconstructive Surgery. 119(3):985-991, March 2007; Kotrikova, Bibiana M.D., D.D.S.; Hassfeld, Stefan M.D., D.D.S.; Steiner, Hans H. M.D., Ph.D.; Hahnel, Stefan M.D., Ph.D.; Krempien, Robert M.D., Ph.D.; Muhling, Joachim M.D., D.D.S.
Facial duplication. The unique case of Antonio. J. Maxillo-facial Surg; Plastic & Reconstructive Surgery. 64(4):571, October 1979; Krupp, Serge
Conjoined Twins; eMedicine Article, Jan 22, 2008; Khalid Kamal MD and others