How to stop printing double-sided pages on macOS

If you're using a Mac and can't find the settings to turn off double-sided (duplex) printing, you can change your default printing options via the following steps.

1. Click the magnifying glass at the top right corner of your screen to open the Spotlight Search bar
2. Type Terminal and press return or click the app from the list
3. Type cupsctl WebInterface=yes and press return
4. Quit Terminal
5. Open any web browser (Safari, Chrome, Brave, Firefox, etc.) from your Mac 
6. Type localhost:631/printers in the address bar and press return
7. Select your printer from the list
8. There are two drop-down menus. Select Maintenance (left) and Set Default Options (right)
9. Navigate through the menus and search for the setting 2-Sided Printing or Duplex. Set the selection to Off and click the Set Default Options button at the bottom to save your changes
10. If prompted, enter your username and password. If that does not work, you may need to enter an administrator's credentials instead.
11. If successful, a screen will validate your changes.
12. Repeat steps 5 to 11 for other printers, if needed.

This was tested and working on macOS High Sierra, macOS Mojave, macOS Catalina, and macOS Big Sur