How do I stop this warning: "W: Duplicate sources.list entry cdrom://Ubuntu 11.10 _Oneiric Ocelot_..."

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http://askubuntu.com – I'm running Ubuntu on a live usb drive and every time I boot, the apt line cdrom://Ubuntu 11.10 _Oneiric Ocelot_ - Release amd64 (20111012)/ oneiric/main i386 Packages and another similar apt line (also sourced from the cdrom) get inserted in the sources.list file. And then I get that warning, which is annoying because I can't proceed with updates and installations unless that warning is rectified. I have noticed that in one of the start-up scripts, the following lines are printed (not sure about the capitalization): Writing new source list Source list entries for this disc are and then th (HowTos)