On 2/14/19 11:37 AM, I wrote:

I have used several different all-in-one printers over the years
(mostly Canon). They all did two-sided printing the same way: print
the odd page numbers first, then ask the user to take those printed
pages and turn them around and put them back in the input paper tray
to have the even numbered pages printed on the back of the odd
numbered pages.

My latest, cheap Canon all-in-one printer, MG3600, does it differently
in a way I can't figure out how it does it!

This new printer when printing double-sided will print page one, then
pull the page back into the printer and print page 2 on the back of
page one. How does it do that?

That's how my ancient HP LaserJet 2200D (basically Canon innards, IIRC)
does it. In the case of certain kinds of paper jam, if you pull out the
tray (top tray, if there's more than one) you get access to a catch that
releases a metal tray into which the pulled-back page has been fed
(printed side down), ready to be fed back through the printer (blank
side down) for the second side to be printed.

Sorry! for "down" read "up."

Mine is rather noisy, with various relays and solenoids doing their
thing to "steer" the paper in the right direction. They may have found a
quieter way by now.