If the printer you selected supports double sided printing you will find the option in the following setting of the print dialog box.
on new macOSs (14,15, and 26) Under the Copy and pages options you will other sections (Printer options, Layout, Paper handling). If you expand the Printer Option you should be able to click the information (i) next to Printer Features. You should now see a "Feature Sets" with a drop down, change it from "General" to "Finishing Options". Then You should see "Print Types" where you can select 1 or 2 sided.
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.