It's not too rare for humans to be born with extra fingers or toes. The condition of having extra fingers or toes is known as polydactyly. It is a congenital disorder, which means that a baby would be born with polydactyly, and did not develop the disorder after he or she was born, and there are several reasons why people may have supernumerary fingers or toes. Polydactyly may also occur in other animals too, including dogs, chickens, mice and cats, usually as a result of mutations to the LMBR1 gene. This gene encodes for a protein that is involved in the development of limbs and as a result mutations (which are changes to the sequence of bases in the DNA) can alter how limbs develop, and therefore cause polydactyly. The extra digit is usually a small piece of soft tissue that can be removed. Occasionally it contains bone without joints; rarely it may be a complete, functioning digit. The extra digit is most commonly an abnormal fork in an existing digit, or it may rarely originate at the wrist as a normal digit does. Polydactyly often occurs on both hands or both feet, or it may occur on just one hand or foot. Similarly, a person may have extra digits on just the hands, just the feet, or some other combination. Although polydactyly is just a mutation of genes. It's natural, and not at all a problem, nowadays, in the industrialized world doctors usually remove the spares during the early childhood of the oversupplied individual. The world record holder for highest number of digits is Akshat Saxena from Uttar Pradesh, India. He was born in 2010 with 7 digits on each hand and 10 digits on each foot, for a total of 34 digits.
I have a friend who was also born with six fingers per hand. At first, I thought that having extra fingers might be an advantage, rather in typing or playing keyboard instruments, but he told me I was crazy. I guess at the very least, you would have to invent new techniques to take advantage of