If the payoff is right, Call=max{0,(S1-S2)-X,(S3-S2)-X}then we can say the payoff is wrong, Call=max{0,(S3-S2)-X,(S3-S1)-X} or Call=max{0,(S1-S2)-X,(S3-S1)-X}because "dual spread " assumes that (S3-S1) is right, (S1-S2), (S3-S2) are mispricing. that's my thought, but I think a little strange.