Change the Duplex Printing Options on a Mac
On the Print dialog window, select the third pull down menu and change Copies & Pages to Layout.
The Layout dialog box will give you the option of turning duplex printing off. It will also allow you to choose the orientation of the duplex printing. Long-edged binding (default) is appropriate for most printed jobs. Short-edged binding however may be desired for documents printed in the landscape (across long edge of paper) orientation.
These setting changes will remain for the duration of your session.