Něco jsem našel:

My config page shows "duplex unit=off" under "print settings". Now if your config page shows this you can enable the duplex using the dos configuration utility that came on the disk with the printer when it was first sold or by using PJL commands. The easiest way to set duplex to on is the "dos configuration utility" but it will not work in windows XP, the software was made long before XP came along. If you have the disk there is a "dos" folder on it with a file called default.pjl. This file has the default setting for duplex "%-12345X@PJL DEFAULT DUPLEX=OFF", the box at the start of the command should be a arrow pointing to the left, if you viewed the file with the dos editor. If you were to find this file and open it with the dos editor, you could change the "off" to "on" then send the "default.pjl" file to your printer using this command "copy /b Default.pjl lpt1" of course you will have to connect the printer to you PC with a parallel cable to do this.
to enter the doe editor type "edit default.pjl" at the command prompt. I would do it in the root directory of C:\ as it makes it easier to find the file after wards.

If this is all way to complicated as it is for alot of users, call HP and do a trade in trade up to a printer with a display and the duplex option by default.