When I set two-sided printing (ubuntu libreoffice 7.3.5.2), and save the printer setting, the setting is promptly forgotten, as can be verified by going back to the printer setting. The fact that the printer is set up in CUPS as two-sided is of no consequence either.
Information pertaining to earlier versions of libreoffice makes reference to a program called spadmin. This program appears to no longer exist. My hope is that by editing psprint.conf, I can force libreoffice to print on both sides, so that one does not need to remember to set two-sided printing each time. I do not find a section 5 manual page for psprint.conf, however. I was hoping to find that information to force two-sided printing by posting this comment.