Change Printers Default Single Sided Setting #mac

 Well, there is an easy way...

http://localhost:631/

This prompted me to do
cupsctl WebInterface=yes
to the terminal... which enabled opening up the printer options... then I could set up the duplex printing.
Went to
http://localhost:631/admin/#General
Then, I had to select "Two-sided, Long-edge binding".

This enabled printing on both sides.