Duogarchy is the outcome of First Past The Post

A true duopoly is a specific type of oligopoly where only two producers exist in one market. In reality, this definition is generally used where only two firms have dominant control over a market. In the field of industrial organization, it is the most commonly studied form of oligopoly due to its simplicity. Wikipedia 28th Dec.'09

First Past The Post results in political duopoly because choices are narrowed so as to avoid a wasted vote.