Assuming that your printer supports duplex printing, it is actually a bit tricky to get OS X to print in duplex mode by default. If you activate duplex mode on your printer via the CUPS web interface (must be activated with the Terminal command "cupsctl WebInterface=yes"; accessed by pointing your web browser to http://127.0.0.1:631) it won't reflect in the OS X GUI. This has to do with the fact that the PPD doesn't reflect changes made to CUPS settings via the web interface.
In order for the OS X GUI to reflect changes made in the CUPS settings we have to make use of the "lpadmin" Terminal command paired along with the "-p" and "-o" flags. For example: "lpadmin -p printer_name -o sides=two-sided-long-edge". By doing it this way, we are altering the PPD that was placed in /etc/cups/ppd/ when the printer was installed.
So, to get a list of printers from CUPS via Terminal use the following command: "lpstat -a"
Once you have a list of CUPS printer name(s) (in my case, "EPSON_WF_7520_Series") you can initiate the command to enable duplex printing: "lpadmin -p EPSON_WF_7520_Series -o sides=two-sided-long-edge".
Now, when printing via the OS X GUI, you will see that duplex mode is selected by default.