copy a dvd onto another dvd

You need to create a DVD of the original video as a stand alone video file, or put it on a USB Flash drive, or Hard drive. When you encode the video, unfortunately that makes it impossible to break the code for copying, except through illegal ripping software. If you have made a video project with iDVD, Toast, or DVD Studio Pro, you can go back to the original project and master new DVDs from the project software that made the DVDs.

Assuming this is a DVD "of your own make," for example originally burned using iDVD, you can duplicate the DVD using Disk Utility. Basically, do the following:

• Insert the original DVD in your Mac. If DVD Player opens, close DVD Player.
• Open Disk Utility
• Select your DVD in the left-hand pane (the DVD, not the DVD drive or a session on the DVD disk)
• Do File > New > Disk Image from <the name of the DVD>
• Save As = name you wish to give to the image file you are about to create
• Image Format = DVD/CD Master
• Encryption = None
• Save

In order to burn new DVD-R media from the disk image you just created, open the disk image in Disk Utility, click the Burn icon and insert a blank DVD-R.

Notes:
The procedure may differ slightly depending on the version of OS X that you have

This proecedure will not work with commercial or copy-protected DVDs.

A little more info is available in this Apple KB article

It is advisable to burn DVD media at lower speeds. I suggest limiting your burn speed to 4x even if your DVD media is rated for faster speeds.

After burning the first copy, make sure it works (in another computer or DVD player) before you burn additional copies.