Thank you for that tip about the Option key. I spent some time solving this earlier today and here's another way: in a web browser, enter http://localhost:631/printers in the address box and press Return. After the list of printers is returned, click on the printer in question. It took me quite a bit of poking around to find the setting for single-sided/double-sided as a default. I suspect one advantage to this technique is that it sets the default in the printer, where the Option key method may only set it for that print queue. Since deleting and re-creating a print queue is a standard troubleshooting step, the web browser technique might be a better long-term solution.

Thank you for that tip about the Option key. I spent some time solving this earlier today and here's another way: in a web browser, enter http://localhost:631/printers in the address box and press Return. After the list of printers is returned, click on the printer in question. It took me quite a bit of poking around to find the setting for single-sided/double-sided as a default (Print Type: under "Finishing Options" tab). I suspect one advantage to this technique is that it sets the default in the printer, where the Option key method may only set it for that print queue. Since deleting and re-creating a print queue is a standard troubleshooting step, the web browser technique might be a better long-term solution.