Pete, and Eric,
Yes hplip is installed on my machine. The version I have is 3.17.11-1. Apparently that hplip contains *.PPD files for multiple HP m452 printers. The problem is some of those printers have duplexers and some don't. But all (both) the *.PPD files have the same nickname, and apparently CUPS uses the nickname to distinguish between "printers". So I found the *.ppd file for m452 with duplexing, changed the nickname, installed that in CUPS, modified the existing m452 printer thingy I already had in CUPS to the new *.PPD and now duplexing, and the printer, are working correctly.
Duplexing shows up in apps that print directly to CUPS, as shown in the attachment, but not in apps that print to the associated PM printer object. I need to figure out how to update the PM printer object with the new PPD file.