My friend's daughter was born with 12 fingers, 6 on each hand. When she's about 9 mths old, she had a minor surgery to have them removed. They had to remove the outer fingers, as they're only mass of flesh - no nerves or bones, I was told. After the removal surgery, now she has to go for a therapy once a week, so that she gets used to using her other (inner) thumbs to grab things.
Apart from that, she looks abosutely fine to me.
Hope this brings you a peace of mind.