Description:
There are two printer options for duplex printing on Linux/CUPS.

One is called "Duplex" and can be set to one of None/DuplexNoTumble/DuplexTumble.
It's described here: https://oip.manual.canon/USRMA-0595-zz-DR-deDE/contents/dlu-cmd-basic-2side.html

The other is called "sides" and can bet set to one of one-sided/two-sided-long-edge/two-sided-short-edge.
It's described here: https://www.cups.org/doc/options.html

It depends on the printer which one is actually used.

I have a Brother MFC-L3730CDN which uses the "sides" option.
I also have a Brother MFC-J480DW which uses the "Duplex" option.

LibreOffice doesn't set the "sides" option which means that the duplex setting in LibreOffice has no effect for printers which rely on this option.
That means the "sides" option is always set to the default value of the CUPS printer for every print job.

To debug this I paused the printer and run the "strings" command on the PPD file of the print job in /var/spool/cups.
It turned out that the "Duplex" setting is set to the value selected in the LibreOffice print dialogue.
But the "sides" option is always the default and never what I selected in the LibreOffice print dialogue.

I want to add that other programs work fine. Only LibreOffice is causing problems.

Steps to Reproduce:
1. Set the duplex default of your CUPS printer to off (one-sided)
2. Reboot your system (necessary; don't know why)
3. Pause your CUPS printer (perhaps not necessary)
4. Print an (empty) two-sided document in LibreOffice Writer with duplex on
5. Run the "strings" command on the PPD file of the print job in /var/spool/cups
6. You will see that it says sides<new line>one-sided

Actual Results:
sides
one-sided

Expected Results:
sides
two-sided-long-edge


Reproducible: Always


User Profile Reset: No

Additional Info:
Version: 7.4.7.2 / LibreOffice Community
Build ID: 40(Build:2)
CPU threads: 4; OS: Linux 6.1; UI render: default; VCL: kf5 (cairo+wayland)
Locale: de-DE (de_DE.UTF-8); UI: de-DE
Debian package version: 4:7.4.7-1+deb12u2
Calc: threaded