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For Ubuntu 9.10 - 12.04

There is an app called Y-PPA-Manager which can do that and much more.

You can install it like that:

sudo add-apt-repository ppa:webupd8team/y-ppa-manager
sudo apt-get upgrade
sudo apt-get install y-ppa-manager

After that, just start the application.

main window

Then select "Scan and remove duplicate PPAs" and hit OK.

advanced window