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MacTheRipper for a direct copy of the DVD's file structure to your hard drive for duplicating or compression, or HandBrake for encoding it to a compressed file format,
 
Since it sounds like you are duplicating a disk that is not a commercial copy-protected disk, it is very simple to do with the program Disk Utility on your Mac without having to involve any additional software.

A quick guide to doing that can be found at http://hathology.com/duplicating_dvds_on_mac_osx/ among many other sources in Google listings.

Hope that helps!
 
Yeah, if it's not encrypted just use Disk Utility to image it, then burn back to blank discs using the restore function.
 
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