The DUPLEX message is used to set the duplex mode for the current printer. The Duplex is a value that is specified in the OIPRINT_EQUATES. Always check the return code from the DUPLEX message for errors, because not all printers support duplex printing.
With horizontal duplex printing, the top of both sides of the page are at the same end of the sheet. With vertical duplex printing, the bottom of one page is at the same end of the sheet as the top of the next page.
errorStatus = Set_Printer( "DUPLEX", duplexType )