You should be able to use Disk Utility (/Applications/Utilities/Disk Utility) to copy the DVD.
If you have a program like Toast (http://www.roxio.com/)
, it can make the copy for you and is easier to use, but you have buy it.
To use Disk Utility:
Put the DVD into the computer.
Start Disk Utility.
Select the DVD in Disk Utility.
Click on "New Image" to create an image of the DVD onto your computer.
Once that is done, eject the DVD.
Open the new image you created and then select it in Disk Utility.
Click burn and put in a DVD to create a copy.
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