A mutation, ofcourse... From FB
This two headed albino milk snake was born last year in Florida.
The condition is known as Polycephaly and occurs when monozygotic twins fail to separate completely. Most organisms with this condition will not live for long, but occasionally you will get cases like this one, where the snake(s?) seem healthy enough and can live for years.
The heads act independently of one another, and will fight over food given the opportunity. Watch a video of their owner feeding them here: http://bit.ly/11YYkHT