If you are using Adobe Reader 10.1.2 - there is an option in the Print dialog box to "print on both sides" of the paper.
This option was only introduced in Adobe Reader 10.1.2 which was released in mid January 2012.
You can prevent Adobe Reader from printing duplex by unticking the option to "print on both sides". After selecting File > Print - the option is about half way down the dialog box.
At the moment this option is ticked by default.
Thanks very much. Your answer was exactly what I was looking for!!!!!!!!!!!!!! Now if I only knew how to change this default but I can live with this procedure.
It should remain default once you have removed it. As far as I'm aware it doesn't check itself again.
Anyway - Adobe are working on a fix for it - which should be around soon... yet another update!