Force always Duplex printing

[Anonymous]

> Add the following to your printer's PPD:
> ---snip---
> *JobPatchFile 1: "% force duplex
>   <</Duplex true>>setpagedevice
>   /_orig_setpagedevice /setpagedevice load def
>   /setpagedevice
>   {
>     dup /Duplex undef % trash any duplex setting
>     dup /Tumble undef % trash any tumble setting
>     _orig_setpagedevice
>   }bind def"
> ---snip---
> Add this around where the "Device Capabilities" are listed.
> It will force duplex printing and inhibit any subsequent
> modification of this key.

I tried this and it works on most printers here. The exception is the HP 4600dn's. It just gets ignored. I tried to google for an answer but came up empty. Is there some other catch? (I'm using the ppd as linked to by linuxprinting.org, btw.)