I think a lot of people struggle with turning off 2-sided printing for network printers. I know I used to struggle with it. Doing this is not hard at all but requires nerd level knowledge (of CUPS, which is what OSX uses for printing) or a simple DIY. Here is the simple DIY:

Launch the Terminal application and type in the following:

cupsctl Webinterface=yes

In Safari enter 127.0.0.1:631 into the browser bar

Go to Administration, Manage Printers, pick the printer, under the Administration drop down pick Set Default Options

Go to General, set 2-Sided Printing to Off, and then click on the Set Default Options button