No duplex printing on Canon Maxify MB5150

Greetings,

recently we had to revive  our printer and scanner  Canon Maxify MB5150.
While it immediately worked flawlessly  from two Windows laptops,  I had
problems with it on a third laptop running Gentoo Linux:

Initially I used  "bjnp://192.168.2.114"  as the printer's URI  but this
lead to repeated messages on the terminal that the colour ink cartridges
are empty and that the job will be restarted in 30 seconds.

That's right,  the colour ink cartridges are empty,  but the PDF file is
greyscale,  and so this should not matter.   Or does CUPS really want to
tell me "You have to buy ink you do not need"?

But since the  CUPS log file  also contained  plenty of earlier messages
"Printer drivers are deprecated  and will stop working  in a future ver-
sion of CUPS"  I changed the URI to  "ipp://192.168.2.114/ipp/print" and
set the CUPS default settings for this printer to "color, 2-sided print-
ing".

At least this solved the problem of the empty colour ink cartridges, but
while the command

   $ lpr -o sides=one-sided TwoPages-GreyScale.pdf

works as expected, the command

   $ lpr -o sides=two-sided-long-edge TwoPages-GreyScale.pdf

does wrap  each printed sheet around,  but without  printing anything on
its rear side.  The second page is then printed on the front side of the
next sheet, the net effect being two identical print outputs.

What am I missing?

Sincerely,
  Rainer