How to fix “Duplicate sources.list entry …” issue

The correct format of repository source line is

<type of repository>  <location>  <dist-name> <components>

For example, a repo line can be like this one

deb http://archive.ubuntu.com/ubuntu precise main

Here, it means, the repository is for binary packages, which are hosted in http://archive.ubuntu.com/ubuntu and
this repository is for Ubuntu precise (12.04) and
this repository contains the main (software which are officially supported by Canonical) component.

Type: The type can be deb and deb-src. deb means a binary repository where deb-src means a source repository

Location: http://archive.ubuntu.com/ubuntu location of the repository.

Dist-name: The distribution name of Ubuntu release. For Ubuntu 12.04 it is precise, for 11.10 it is oneiric.

Component: It can be main, universe, multiverse and restricted.
These words indicates the level of supports for the packages and the licensing status.

See this page for more information.

Please take note that, you can add one or more component in a line,
so “main”, “universe”, “restricted” and “multiverse” can be in a single line. Also note,
Though you add more than one component in a single line,
APT system considers them as seperate line containing only one component.