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