I have tried the below code on multiple PCs and printers (HP Inkjet and Toshiba Studio) and neither printer prints duplex - both are capable of duplex and will print on both sides if Printer Preferences set
DoCmd.OpenReport Report_Name, acViewDesign, , , acHidden
With Reports(Report_Name).Printer
.Orientation = acPRORPortrait
.ColorMode = acPRCMMonochrome
.Duplex = acPRDPHorizontal
End With
DoCmd.OpenReport Report_Name, acviewPreview