Duplex is a special twinning stipulation. It states that there is
another (twin) problem obtained by reversing Wh. and Bl. roles. E.g.
a "mate in 2, Duplex" is a mate in 2 for White and a mate in two for
Black (assuming now that Black has the move).
Werner Keym, Die Schwalbe 224, June 2007,
Bernd Schwarzkopf, Die Schwalbe 227, October
2007, No. 13514
Cuppini, Die Schwalbe
168, December 1997, No. 9856
Dr. Tomislav Petrovic,
StrateGems 10, H0313
F. Kapustin, Shakhmatnaya
Kompozitsiya 49, No. 286