Also reproduced on ubuntu 14.04 with HPLIP 3.14.4. The duplexer initially worked with gutenprint + the system provided HPLIP 3.14.3 Officejet 8600 PPD, but I installed 3.14.4 so the scanner would work. I then uninstalled HPLIP 3.14.4, re-installed the HPLIP 3.14.3 from apt, and tried gutenprint again and now it won't duplex either.
Also reproduced on ubuntu 14.04 with HPLIP 3.14.4. The duplexer initially worked with gutenprint + the system provided HPLIP 3.14.3 Officejet 8600 PPD, but I installed 3.14.4 so the scanner would work. I then uninstalled HPLIP 3.14.4, re-installed the HPLIP 3.14.3 from apt, and tried gutenprint again and now it won't duplex either.