If the printer you selected supports double sided printing you will find the option in the following setting of the print dialog box.
On the latest macOS (14,15, and 26) first click on the Printer... button inside the print dialog box. In the Print window, below the Copy and Pages options you will other sections (Printer options, Layout, Paper handling). Expand Printer Options and you should be able to click the information (i) icon next to Printer Features. You should now see a "Feature Sets" with a drop down, change it from "General" to "Finishing Options". Below now you should see an option for "Print Types" where you can select 1 or 2 sided. Remember to click OK, then Print.
If you are on an older macOS and/or do not see the above options you might have them listed separately or under Layout in the setting drop down box. Look for a Two-Sided option after switching Copies & Pages to Layout.