I have messed up the PPA and now I do not know how to revert the sources.list to its original form.
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Check that you don't have any elementary OS repositories launching Software & Updates from Slingshot or terminal:
If you have checked it, open a terminal and execute:
sudo add-apt-repository ppa:elementary-os/os-patches sudo add-apt-repository ppa:elementary-os/stable sudo apt-get update
Try to check now for new updates with:
sudo apt-get dist-upgrade