I've just figured out about the cpdftocps filter...
Deleting "sides" from the list of options not to send to pstops fixes
this bug.
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pstops filter ignores sides=two-sided-* options for duplex printing
https://bugs.launchpad.net/bugs/411392
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Public bug reported:
Binary package hint: cups
Attempting duplex printing using standard documented duplex options
doesn't produce duplex jobs.
% zcat /usr/share/doc/cups/changelog.Debian.gz | lp -d printer -o sides
=two-sided-long-edge
Resultant postscript output contains this...
===
%%Be