Sometime since buster, but probably bullseye, my Dell 2130cn stopped printing
in duplex. I don't use the printer much and I upgraded from buster to bullseye
to bookworm in fairly quick order, so I don't recall if I ever tried printing
in bullseye.

The pxlcolor driver is the only driver that prints in color for this printer. I
used to be able to use this driver and print in duplex with this printer. Now,
the printer no logner prints in duplex even though all the options are set to
make it set that way.

I was able to print in duplex from Windows, so I decided to try a different
driver. I tried both
"Generic PCL 6/PCL XL Printer - CUPS+Gutenprint v5.3.4"
and
"Generic PCL 6/PCL XL Printer Foomatic/hpijs-pcl5c"

They both print in duplex, but do not print in color. I would have thought that
hpijs-pcl5c would have worked in color.

I have had the same problem with a FujiXerox Docucentre SC2020. Being a
little new, I have been trying to understand the bug reporting system to
make a coherent submission, but will add these notes here. Please tell
me if this is the wrong way to go about it. I'm also not sure what the
relevant package is, if it is this one or some other.

The SC2020 will print duplex from Ubuntu 18.04.6 LTS and from Raspbian
GNU/Linux 10 (buster). It is not the PPD file changing, because copying
the working pxlcolor.ppd from those machines makes no difference.

I changed to debian bookworm after the previous machine died, so this is
a fresh install.

One point I have noticed, that may or may not be relevant, is that the
printer seems to receive a duplex command of some sort: on the printer's
LCD display, when looking at the details of a duplex job, it shows
"two-sided" even though it only prints on one side.

Another possibly irrelevant point is that this is over ethernet, and the
problem persists whether it is IPP, socket 9100, or LPD.

I have also tried
     'Generic PCL 6/PCL XL Printer Foomatic/pxlcolor (recommended)'
and
     'Generic PCL Color Laser - CUPS+Gutenprint v5.3.4'
and some others, in a fit of confusion, as drivers; these also fail to
print duplex.