Hi mikelester,
Thank you for using the Support Forum. Please take a look at these solutions for Unable to Duplex and see if any of these will address the issue you are seeing. If not please consider contacting your support centre for further assistance.
I just installed a 6700DN and when we go to print, the 2 sided print option is gray and can't be chagned/selected. I am running:
Windows 7 Home Premium 64-bit
Printer driver 5.210.15.0N
Need help please!