Unable to Duplex Print from Mac 10.5 using InDesig

InDesign's knowledge base says

Duplex printing
Printer-specific features such as duplex printing are available when you click the Printer button in the Print dialog box. Duplex printing is available only if the printer supports it. For information on duplex printing, see your printer documentation.

If you acquired the driver through Mac's library, you may want to download and install the latest driver after installing the ppd.

Cut and pasted from the Adobe forums:

I upgraded to InDesign CS4 a month or so ago.  I'm using a 2.8 GHz Quad-Core Intel Xeon, Mac OS X 10.5.7, and having trouble printing to two different printers.  I had a margin issue with an HP Color Laser 5550, where I never had one before.  Then today, I discovered that I could not duplex print to a HP Laser 3005, again this was never a problem with CS3.
 
So needless to say this thread was required reading for me.
 
Of course, the pdf work-around worked.  But, I wanted to get to the bottom of this.  Well, putting together info from a couple of different posts, I think I've discovered the duplexing problem (at least).
 
In the past, I have religiously used the Page Setup tab in the printer window to set paper size, and when I did so the Set-up Paper Size would automatically show "Defined by Driver."
 
Well, now, one can follow the same process (setting the paper size in the driver via Page Setup), and when one returns to the main printing menu, the Set-up Paper Size "window" will not have changed, in other words, it will not say "Defined by Driver" but rather "Letter" or "Legal."  When I manually changed the setting to "Defined by Driver," lo and behold, I could duplex!
 
So the theory espoused above about whether the printer or InDesign was controlling the printer does seem to be the key issue.  If you're using garden-variety desktop printers, you're most likely emulating Postscript, and the InDesign is expecting Postscript accuracy and not finding it.