Thanks for your intention to help. Much appreciated. In the end I resolved the issue by following the steps described in this other forum post:
Is it a 6515, 6515N, or a 6515DN?
Only the 6515DN has auto-duplexing. If your unit is a 6515 or 6515N, it will not duplex - except by hand (i.e. print side one and reload the page then print side two)
I have just purchased a new Xerox 6515 DNI printer and I installed it on our home wifi network. I can print properly from my computer, but if I want to print on both sides of the page, that option is grayed out:
Any idea on how to enable that?