k9Copy

Overview
k9Copy is software for copying and re-encoding dual-layer (8.5 GB) DVDs using a lower-quality compression in order to fit on a single-layer (4.7 GB) disc.

It apparently requires the Medibuntu package in order to read encrypted DVDs.

Reference

 * project homepage
 * Community Ubuntu Documentation
 * Community Ubuntu Documentation

Guides

 * How to copy DVD9 to DVD5 using k9copy