What are the exact dimensions of a DUPLO brick?
For the short answer, Duplo studs are indeed taller than twice the height of a Lego stud.  And don't rely too heavily on measurements you get from ldraw (and not just for Duplo).

For a really long-winded history on this...

First, note that when I authored the original Duplo parts I never actually measured them with a ruler or calipers, as there was no need.  The ldraw system assumes "perfect" dimensions in terms of other ldraw parts, so you typically measure parts by comparing them to other Lego pieces.  By this measurement, Duplo in ldraw (not counting the studs) is exactly twice the size of Lego.  In real life, any part is slightly smaller than the nominal measurement so that you can slide two parts next to each other without them hanging up on each other.  Duplo parts seem to be about twice as tolerant in this way, so I'd guess that in real life also the outer dimensions are very close to twice the size.

Some time in the 1990's, Duplo studs were changed.  They were made taller, and the inner hole got larger (making the stud walls thinner).  They also added bumps to the inside of the stud hole.  The ldraw Duplo stud has the taller height of the new stud and the perfect roundness and dimensions of the old stud hole.

Regular Lego bricks used to have smooth internal walls, and the ldraw bricks still look like this.  Duplo bricks always had "splines" that run vertically along the inside walls for Duplo studs to clutch.  Regular Lego studs clutch against the outer wall of the Duplo brick instead.  This is how studs of different sizes can both fit into the underside of a Duplo brick.  Around (2003?) Lego started making the walls of regular Lego look like Duplo bricks.  I assume this is a cost-cutting measure where they now use less plastic to mold the walls, but now have splines like Duplo so that the walls are the old thickness in the places where studs need to clutch.

So... Duplo wall thickness is the same as Lego wall thickness from before 2003 or so.  It's 4 LDU (ldraw units) if that helps.  For reference, a plate is 8 LDU high without the stud, and a 1x1x1 brick is 20x20x24 LDU without the stud.  Studs in ldraw are 4 LDU high, but that is totally inaccurate.  Non-hollow studs are clearly more than half the height of a plate, and the real height of regular studs should be more like 4.25 LDU.

Be very wary of the measurements that you get from parts converted from ldraw format to POV-Ray.  L3P scales each part down just a little bit to let POV-Ray show edge lines.

There is no simple real-world conversion from ldraw units to inches or centimeters.  For more info on that, see my lugnet post from 2001: http://news.lugnet.com/cad/?n=5284.

Primo is another story...