Duplicate sources error while updating

Question

I am a beginner with Ubuntu and any help is appreciated. When I run sudo apt-get update on Ubuntu 12.04 LTS I obtain at the end the following message:

Reading package lists... Done
W: Duplicate sources.list entry cdrom://Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)/ precise/main i386 Packages (/var/lib/apt/lists/Ubuntu%2012.04.2%20LTS%20%5fPrecise%20Pangolin%5f%20-%20Release%20amd64%20(20130213)_dists_precise_main_binary-i386_Packages)
W: Duplicate sources.list entry cdrom://Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)/ precise/restricted i386 Packages (/var/lib/apt/lists/Ubuntu%2012.04.2%20LTS%20%5fPrecise%20Pangolin%5f%20-%20Release%20amd64%20(20130213)_dists_precise_restricted_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

Output of sources.list:

deb cdrom:[Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)]/ dists/precise/main/binary-i386/

deb cdrom:[Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)]/ dists/precise/restricted/binary-i386/
deb cdrom:[Ubuntu 12.04.2 LTS _Precise Pangolin_ - Release amd64 (20130213)]/ precise main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://it.archive.ubuntu.com/ubuntu/ precise main restricted
deb-src http://it.archive.ubuntu.com/ubuntu/ precise main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://it.archive.ubuntu.com/ubuntu/ precise-updates main restricted
deb-src http://it.archive.ubuntu.com/ubuntu/ precise-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://it.archive.ubuntu.com/ubuntu/ precise universe
deb-src http://it.archive.ubuntu.com/ubuntu/ precise universe
deb http://it.archive.ubuntu.com/ubuntu/ precise-updates universe
deb-src http://it.archive.ubuntu.com/ubuntu/ precise-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://it.archive.ubuntu.com/ubuntu/ precise multiverse
deb-src http://it.archive.ubuntu.com/ubuntu/ precise multiverse
deb http://it.archive.ubuntu.com/ubuntu/ precise-updates multiverse
deb-src http://it.archive.ubuntu.com/ubuntu/ precise-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://it.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://it.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu precise-security main restricted
deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
deb http://security.ubuntu.com/ubuntu precise-security universe
deb-src http://security.ubuntu.com/ubuntu precise-security universe
deb http://security.ubuntu.com/ubuntu precise-security multiverse
deb-src http://security.ubuntu.com/ubuntu precise-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main