Ulnar dimelia
Ulnar dimelia, also referred to simply as mirror hand, is a very rare congenital disorder characterized by the absence of the radial ray, duplication of the ulna, duplication of the carpal, metacarpal, and phalanx bones, and symmetric polydactyly. In some cases surgical amputation is performed to remove the duplicate carpals, metacarpals and phalanges. As of 2015, approximately 70 cases have been recorded in the medical literature.
Bone deformity may also accompany nervous and arterial anomalies in some cases due to the duplication of the ulnar nerve, the presence of abnormal arterial arches, the duplication of the  ulnar artery, the shortening of the radial nerve, and the absence of the radial artery. The diagnosis of ulnar dimelia is based on laboratory tests of frontal and sagittal planes in individuals suspected of the condition. There are two types of ulnar dimelia noted in medical journals: Type 1 ulnar dimelia entails one lunate and one trapezoid bone as well as one index finger, while type 2 ulnar dimelia has two lunate and two trapezoid bones as well as two index fingers.
The American Society for Surgery of the Hand and the International Federation of Societies for Surgery of the Hand classified ulnar dimelia in the third group of congenital hand deformities in accordance with the characteristics proposed in the Swanson classification (1976).

References
Tomaszewski, Ryszard; Bulandra, Andrzej (2015). "Ulnar dimelia-diagnosis and management of a rare congenital anomaly of the upper limb". Journal of Orthopaedics. 12 (Suppl 1): S121–S124. doi:10.1016/j.jor.2015.01.027. PMC 4674541. PMID 26719621.
Namdev, Rupesh. "Ulnar dimelia | Radiology Reference Article | Radiopaedia.org". radiopaedia.org.
Chinegwundoh, J. O. M.; Gupta, M.; Scott, W. A. (1 February 1997). "Ulnar dimelia: Is it a true duplication of the ulna?". Journal of Hand Surgery. 22 (1): 77–79. doi:10.1016/S0266-7681(97)80024-1. PMID 9061533. S2CID 31636571.
Jameel, Javed; Khan, Abdul Qayyum; Ahmad, Sohail; Abbas, Mazhar (2011). "Ulnar dimelia variant: a case report". Journal of Orthopaedics and Traumatology. 12 (3): 163–165. doi:10.1007/s10195-011-0146-y. PMC 3163772. PMID 21769660.