"Mirror hand" is a real, documented congenital condition.
Whether these particular images showing the syndrome are genuine.
In April 2018, a set of images purportedly showing a hand with eight fingers and no thumbs started to go viral:
Although these images were met with a heavy dose of skepticism, we found several reasons to believe that these photographs are genuine. At the very least, they do depict a real syndrome.
Many Internet users may have encountered these images on Imgur, where they were posted with a humorous title, or various Facebook pages, where they were shared with some misinformation about “mirror hand syndrome.” These unreliable posts may have made some viewers especially doubtful about its authenticity.
However, while we cannot say with authorities that these exact photographs are real, they do show a real condition: Ulnar dimelia, a rare congenital anomaly of the upper limb characterized by “duplication of the ulna, absence of the radius and polydactyly”:
Ulnar dimelia, called also the mirror-hand, is classified to the 3rd group of congenital hand malformation, according to the classification proposed by Swanson [Swanson 1976] and adopted by the American Society for Surgery of the Hand (ASSH) and the International Federation of Societies for Surgery of the Hand (IFSSH).
There have been several documented cases. Dr. Rupesh Namdev of Bilaspur, India documented a similar condition in a seven-year-old girl in 2012. This child also had eight fingers on one hand:
Plain radiograph of wrist joint shows polydactyly with duplication of ulna, duplication of ulnar side of carpal bones, metacarpals and phalanges, absence of radius and absence of thumb. Eight triphalangeal digits are seen. The middle phalanx of medial index finger is hypoplastic.
The condition can be mitigated with surgery.