Im using cs5.5 and i can't find any option in the print window to switch double sided printing on/off.
The default printer settings on my Mac have already been set up for 1 sided printing.
That setting is coming from the ppd assigned in your print driver. Every printere is different but your lcoation should be similar to mine.
Print >> Printer Button >> Layout to Printer Features (or similar) >> Duplex (Or double sided printing)
You may also be able to change this feature on the back end if you have a stand alone RIP(change the default settings for files received), or in the menu display panel button on the printer.