I just got an Epson WF-3520DWF. I also could not get duplex to work at all. The driver was epson-inkjet-printer-201212w_1.0.0-1lsb3.2_i386.deb downloaded from http://download.ebz.epson.net/dsc/searc ... archModule
Now it sort of works...
In CUPS, I selected the printer, then from the Administration dropdown I selected Set Default Options.
Then the bit that was not mentioned before in this thread - there are links across the top of that CUPS page, "Paper & Quality Options", "Reduce/Enlarge Document", "Auto Duplex Printing", "Multi-Page", etc. Click on "Auto Duplex Printing". Now there are 2 options: "Duplex Printing - Binding Edge:" and "Adjust Print Density:".
I selected one of the "on" options in the "Duplex Printing - Binding Edge:"
Now EVERYTHING comes out duplex. That's not so good either. If I am in LibreOffice, and I select File|Print, select the Epson printer, click the Properties button, I can select Duplex on (top or side) or off, and it makes No Difference At All!!
Am wondering where to turn next...
(Haven't tried the scanner in the printer yet, that will be another day's fun)