Dissociative identity disorder
Within a person there exists two or more distinct personalities or personality states each having its own unique memories, behaviour patterns and social relationships. At least two of the personalities take full control of person's behaviour and transition may occur suddenly or gradually, over several hours or days.
The implications of the problem are seen in the extreme case of a 1990 trial in the USA in which a single witness was sworn in 3 times to allow different personalities to testify. Psychiatrists believed that a further 43 personalities inhabited her body.