I had exactly the same problem as the original poster. Other laptops and mobile printing duplex worked fine. Mine did not (but was printing duplex the previous day).
Followed the Device settings and it kept reverting back to "manual" when trying to print.
CHANGING THE PORTS eventually worked, but I had to go through each of the ports on the list to find which one worked, so it may NOT be the TCP/IP port. Try one of the other ports available first.
Mine was on LPT3: Printer port.