If a surface f \in S^{3} admits singularities but its Gauss map is globally defined and can be smoothly extended across singularities, f is called a frontal. A frontal is called extrinsic flat (or E-flat) if its Gauss map degenerates everywhere. The representation formula for E-flat great circular frontals is defined by using Legendrian duality developed by Chen and Izumiya. As applications, we give the classification of singularities and introduce the notion of the conjugate of E-flat great circular frontals, which exhibit the duality between SW and CCR. Moreover, we investigate the dual properties of conjugate frontals, including the relationship between their representation formulas, the connection between conjugation and Legendrian duality, and dualities of other singularities.