I had the problem the other way around - i.e. my Xerox Versalink C400 printer wouldn't print double-sided. I phoned Xerox support and the agent told me it was because I was using a USB cable. This struck me as odd so I did some more Googling and found https://www.support.xerox.com/en-us/article/en/2113917 - I had to manually set the Duplex Unit to "installed" in Printer Properties/Device Settings. After that in Printing Preferences/Earth Smart the "2-Sided Print" checkbox was ticked.
Perhaps if you use an ethernet connection as opposed to USB, maybe the PC can use SNMP to determine whether the printer is duplex-capable, or some such malarkey. But the agent should have been able to advise me of this very simple fix.
Does it print duplex from all applications? I would check your application menu first. I found the steps on how to duplex print, so check the steps to make sure you have it turned off. If you really want to turn it off permanently, you could disable duplexing as an installable option from the driver.
I've just purchased a C400 for use in our office. I'm unable to disable Duplex printing on it though. I've changed the driver option to 1 sided print and have also disabled 2 sided in Earth Smart. I've tried the Postscript and PCL6 drivers but nothing seems to make a difference, everything prints duplexed. Just wondering if I'm missing anything obvious please?