[MnM]

You can use Disk Utility for this.

1) Insert the DVD into your optical drive. When DVD player starts to play the movie, quit DVD player and proceed to open Disk Utility.

2) Select the dvd from the list of other disks.

3) Next choose File > New > "Disk Image from (name of disc)."

4) Then choose a destination, name the file, and I usually leave mine at compressed with no encryption and hit save.

5) After the image is made, eject the orginal dvd out of your computer.

6) Select the disk image, which should now be in disk utility's list, and click burn.

7) It will prompt you to insert a blank dvd. After doing so hit burn, give it some time and presto!

This and more can be found by opening up Disk Utility, clicking "Help" from the top and choosing "Disk Utility Help". In there search for "duplicating a CD or DVD".