Printer is a HP Laserjet Pro M478. Computer is a MacBook Pro running the most current software.
Printing from Reader always prints duplex, ignoring the setting in the print dialog "print on both sides of paper". No presets or print options related to duplex printing are available, save the "print on both sides of paper" checkbox.
On checking, I am having the same problem in Lightroom, which includes the opportunity to pick single sided presets (which are also ignored). All of my non-Adobe products (eg Microsoft Office, Mail, Safari, Chrome, Apple Photos, Calendar, etc) are working correctly, eg duplex when duplex is chosen, single-sided when it is not, so it doesn't appear to be any sort of issue with the printer or system side drivers.