I remember seeing in a Chinese documentary TV show an old American with 6 fingers on each hand (each with individual joints). Unfortunately, he's not a pianist. But when he tried putting his hands on the piano keyboard, he can span 14s (as in from DO to the TI of the next octave - that's almost 2 octaves in just one hand! Do you any pianists with this abnormality and is such an advantage or disadvantage? BTW, here are some pictures...https://www.effected.wronger.com/image/12fingers.jpghttps://www.sonypictures.com/tv/shows/ripleys/database/ep_313a.html
I remember hearing of a pianist who had a span of two octaves, and wrote music impossible to play by anyone else, but I don't think with 12 fingers.
Rachmaninov?
I don't think it's a very uncommon genetic defect to have 6 fingers on each hand - it's not so common that I personally know anyone with so many fingers, but in science class in high school it was mentioned as one of the more common genetic conditions (I hardly ever went to the biology/life science lessons, so this must have been one of the rare occasions so I remember it!)