Self-splitting is a psychological defense in which your mind reacts at opposite states of consciousness to protect your internal world. Like an armor, defenses protect the mind when it is being attacked by a life experiences. They are necessary for your survival, they do not reflect you are “crazy”, it just happens unconsciously, without awareness... None of this is your fault.
As I know, Duality refers to having two parts, often with opposite meanings. The duality of good and evil, peace and war, love and hate, up and down, black and white. If there are two sides to a coin, metaphorically speaking, there's a duality in Self Splitting.