So there is a World Cup story about a Brazilian family who has 12 fingers (6 on each hand) spreading cheer, bringing luck, etc... The title of many articles about it read something like "odd" and "rare genetic order".
However, I am not sure why we consider it this way. Polydactylism is actually a genetically dominant trait and will actually be visible as a physical trait for someone that has the gene. Most people that have it, actually have the supernumerary removed at birth.
So this really comes down to a question of selection versus dominance. So if it is so 'easy' to pass this trait on, why don't we see more of it? The question comes down to selective advantage (Hellllooooo Darwin). It could be possible that animals that have it are somehow limited. In humans, perhaps its not an attractive feature, so those with it mate less.
But realistically, lets face it, here's the real reason: My name is Inigo Montoya, you killed my father, prepare to die.