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 the above apt lines.

I don't know this is a lead to what's causing the duplicate entries. Also, I don't even want any cdrom to be in the sources.list file. Everytime I delete them whenever I get that warning when attempting to update/install and everytime I reboot, they come back again. Your help is appreciated.