Set Printer Duplex

uses printers, winspool

//

function PrinterSupportsDuplex: Boolean;
var
  Device, Driver, Port: array[0..255] of Char;
  hDevMode: THandle;
begin
  Printer.GetPrinter(Device, Driver, Port, hDevmode);
  Result :=
    WinSpool.DeviceCapabilities(Device, Port, DC_DUPLEX, nil, nil) <>
    0;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  Device, Driver, Port: array[0..80] of Char;
  DevMode: THandle;
  pDevmode: PDeviceMode;
  begin
     // Get printer device mode handle.
    Printer.GetPrinter(Device, Driver, Port, DevMode);
    if Devmode <> 0 then begin
       // lock it to get pointer to DEVMODE record
      pDevMode := GlobalLock(Devmode);
      if pDevmode <> nil then
      try
        with pDevmode^ do begin
          dmDuplex := DMDUP_VERTICAL;
          dmFields := dmFields or DM_DUPLEX;
        end;
      finally
        // unlock devmode handle.
        GlobalUnlock(Devmode);
      end;
    end; { If }
end;