Sometimes, you get a new printer with a duplex feature, and your Mac, for whatever reason, decides that it should print on both sides of the page by default. (I think this is a problem with the HP printer driver installer, actually.)

Fixing this is doable, but…not obvious. At all. Here’s what you do:

1. In the Finder, click the “Go” menu (four over from the Apple menu)
2. Choose Utilities
3. In there, open Terminal
4. At the prompt, type “sudo cupsctl WebInterface=yes” and press return (without the quotes)
5. Type your computer password (you won’t see it as you type) and press return
6. Terminal should simply respond with a prompt like the first prompt, and nothing else. Quit Terminal.
7. In Safari or another browser, go to http://localhost:631
8. You should see a screen titled “CUPS”. Click the “Printers” tab. (If you get asked for a password, use your computer user’s short user name, and your usual computer password. If you don’t have the short user name, see this Apple Support article.)
9. Click your printer in the “Queue Name” column
10. In the rightmost of the two dropdowns, choose “Set Default Options”
11. I don’t know exactly what options you’ll see next, but one of them should pertain to 2-sided printing. Set it to “Off”.
12. Click the “Set Default Options” button.
13. You’re done; you can close the browser window.

H/T to the helpful people in this Apple Support thread.

Photo by tom_bullock, courtesy Flickr Creative Commons.