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.
Thanks for your reply,
After checking this setting it seems to be printing 1 sided now.