This would simply be a driver setting or incorrect driver issue, but you don't list what driver you have, and there is as far as I know, no such thing as a Phaser 6115-DNI. I can't even suggest a proper driver since you also did not mention that Operating system you are running where this issue occurs.
If it is a Windows Operating system, I would suggest the latest GPD-PS. Installed manually, so if in Win 8-10, do not select it from the list when installing, choose what is circled here
And at that point, if it still doesn't give duplex as an option just manually add the duplex option, to do that go to Control Panel > Devices and printers
Right-click the printer and go to Printer Properties > Configuration and enable the duplex with the dropdown box
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for posts that resolve your issues. Your feedback counts!Joe Arseneau