The DUPLEX message returns the current duplex setting for the printer. See the OIPRINT_EQUATES for the Duplex values.
duplex = Get_Printer( "DUPLEX" )
The duplex setting of the current printer.
1 = Duplex Simple
2 = Duplex Horizontal
3 = Duplex Vertical
declare function duplex = Get_Printer( "DUPLEX" )