Last week I had reason to duplicate some video DVDs. Yes, it was legitimate; I wanted copies of personal video footage to share with family. After fiddling around a bit I found a very straighforward way to do this task using simple GNU/Linux tools.
The first step is to copy the DVD to the hard drive (I only have one DVD drive).
cat /dev/sr0 > dvd.iso
where /dev/sr0 is my DVD device.
Then, after changing to a blank DVD, simply
growisofs -dvd-compat -Z /dev/dvd=dvd.iso
and it worked.