I figured out an answer to my question.
On the MAC, running OSX 10.15, when you select "Print" from the File menu a print dialog box opens. Here you can customize a variety of settings for printing a selected document. Among the settings is an option for 2-sided printing.
Most important, though, is the choice of "Presets." Here you can use the current group of settings as the default for the printer.
So to solve the problem, uncheck the "2-sided printing" box, create a Preset for all the current settings, and save the result with a custom name. Now select this Preset as the Default and it will be used for all future printing.
