Polydactylism--being born with more fingers and/or toes than you're supposed to--is a pretty sweet deal. You can do awesome stuff if the extra digits are functional, and have them easily removed if they're not.
Akshat Saxena holds the Guinness World Record for Most Fingers and Toes. He has 34 digits total.
It's pretty common, seen in about 1/500 live births.
It can be isolated or just a symptom of several way worse genetic diseases.
Pretty much nothing. The surgery to remove it is usually uncomplicated and done purely for aesthetic reasons and shoe fittings. Parents of children with extra toes usually cite the latter as the main reason for having their kid get the surgery.
Kamani Hubbard, a baby in San Francisco, was born with a completely functional extra digit on each hand and foot. His dad thinks it's pretty cool, but his mom is "in shock"--mainly because not a single fucking doctor noticed, at any point in the baby's development. Even when Kamani came bouldering out of his mom's womb, the nurses told her the baby was "perfect."
"Well obviously we meant besides that."