I have got a DJ 5652 printer with duplex option. If the printer is
connected directly to the OV everything works finde. Now I have
connected the printer to my DSL router (AVM FritzBox). But since then it
is no longer possible to print double-sided with the duplex option.
There are different problems depending on the operating system:

Windows 2000:
The properties dialog for the printer shows that the duplex options is
available and that the printer can print double-sided but in the printer
settings I do no longer see the options for double-sided printing. I
have installed the latest drivers from the HP web site (network version)
but this did not help.
Maybe there is registry tweak to change the behaviour?

On Vista the options for double-sided printing are available in the
printer settings dialog but the pagination is not correct. The page is
flipped on the small side of the page. The respective setting is
completly ignored.

Any ideas how to fix this?