Bug#1071595: cups refusing to see duplex printing functionality when adding HP m479fdw printer

[Jonathan Kamens <jik@kamens.us>]

I am now seeing this issue with my HP m479fdw printer.

Two computers, computer A with the printer added through the settings 
app back in July, computer B with the printer added today. In both cases 
the printer was added exactly the same way. On A, duplex printing is 
available. On B, it's not.

I can get duplex functionality on B as follows:

1) Shut down cups.

2) Delete /var/cache/cups/*.*

3) Replace the PPD file for the printer in /etc/cups/ppd with the PPD 
file from A.

2) Start cups back up.

Now duplex printing works.

Note: if I remove the printer on A (after backing up the PPD file) and 
re-add it, duplex printing goes away. I can then restore duplex printing 
as described above.

So the problem is clearly that when the printer is initially added CUPS 
is failing to detect the fact that it supports duplex printing. Not only 
that, but this is a regression, otherwise I wouldn't have access to the 
PPD that was generated on A back in July that has duplex options in it.