Shakespeare Programming Language, 204 bytes

A.Ajax,.Puck,.Act I:.Scene I:.[Exeunt][Enter Ajax and Puck]Ajax:Open mind.Puck:Open mind.Is I nicer than zero?If not,Let usScene V.Is you as big as I?If so,Let usAct I.You is zero.Scene V:.Puck:Speak thy. 

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Outputs the null character if input is not double speak, and outputs nothing (exits with error) if it is double speak.