Sounds like the Generic PS driver is in use even though you installed the right one.
Check here: (If it doesn't list the model it isn't the right driver)
I tested mine on 10.11 and had no issue using duplex.
Running MacOS 10.10. Duplexing function worked fine for long time, then suddenly not working. I have tried the following:
1) Restarting the printer
2) Updating driver to latest available
3) Tried duplexing in multiple applications, such as Microsoft Office, Acrobat, and Mail. All allow me to "choose" 2 sided duplex, which I do, but it is printing on two separate sheets.