I keep getting this warning whenever I try to run sudo apt-get update .

W: Duplicate sources.list entry http://archive.ubuntu.com/ubuntu/ precise-updates/main i386 Packages (/var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_precise-updates_main_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

Below is the output from /etc/apt/sources.list file.

deb http://archive.ubuntu.com/ubuntu precise main restricted
deb-src http://archive.ubuntu.com/ubuntu precise main restricted

deb http://archive.ubuntu.com/ubuntu precise-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu precise-updates main restricted

deb http://archive.ubuntu.com/ubuntu precise universe
deb-src http://archive.ubuntu.com/ubuntu precise universe
deb http://archive.ubuntu.com/ubuntu precise-updates universe
deb-src http://archive.ubuntu.com/ubuntu precise-updates universe

deb http://archive.ubuntu.com/ubuntu precise multiverse
deb-src http://archive.ubuntu.com/ubuntu precise multiverse
deb http://archive.ubuntu.com/ubuntu precise-updates multiverse
deb-src http://archive.ubuntu.com/ubuntu precise-updates multiverse

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

What should i do to fix this problem?

Best Answer

Your sources.list really has duplicate entries.

Let's first see the correct form of a source line or repository line

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.
^{You can visit the Ubuntu Wiki to view an updated list of Ubuntu releases and their code names.}
Component: It can be main , universe , multiverse and restricted . These words indicate the level of supports for the packages and the status of the licensing

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 separate line containing only one component.

So, If your sources.list have a line like this

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

Then it can't have another line like the below (which your files have)

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

which is equivalent of these two lines

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

Because, you are duplicating universe twice, so there will be an error for that duplicate. For every duplicate found an error will be given

Solution:

After analyzing your sources.list file, I found that, It is a basic one which is supplied by Ubuntu by default. You can build a default file following the procedure

Open a terminal and first rename the existing sources.list file to sources.list.bak file. (we can safely remove that, but caution is good).
```
 sudo mv /etc/apt/sources.list /etc/apt/sources.list.bak
```
Then open software-properties-gtk to build a new one. Type the below command in a terminal or in dash command prompt with pressing Alt+F2
```
software-properties-gtk
```
^{Note, for Ubuntu version 11.10 or earlier you have to use gksu before the software-properties-gtk command}
After the window is open,
- Select all four categories in Ubuntu Software tab
- Select precise-security , precise-updates and precise-backports category in Updates tab.
- Select Canonical partner and Independent category from Other software tab.

That's it You have now a default sources.list file without error. Compare this file with the previous one if you want

Update to deal with `sources.list.d` dir's files

Sometimes a duplicate entry can be in a file in the /etc/apt/sources.list.d directory, which is also used by APT. So you should look at the directory and see if there is any duplicate in these files It's not necessary to put duplicates in the same file

Example case:

A user had this error message showing while doing sudo apt-get update .

W: Duplicate sources.list entry http://dl.google.com/linux/chrome/deb/ stable/main amd64 Packages (/var/lib/apt/lists/dl.google.com_linux_chrome_deb_dists_stable_main_binary-amd64_Packages)
W: Duplicate sources.list entry http://dl.google.com/linux/chrome/deb/ stable/main i386 Packages (/var/lib/apt/lists/dl.google.com_linux_chrome_deb_dists_stable_main_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

But there was no entry with http://dl.google.com in the main /etc/apt/sources.list file. Looking at the /etc/apt/sources.list.d/ directory we found these files.

output of ls /etc/apt/sources.list.d/ .

aims-sagemath-precise.list
aims-sagemath-precise.list.save
dropbox.list
dropbox.list.save
fossfreedom-packagefixes-precise.list
fossfreedom-packagefixes-precise.list.save
google-chrome.list
google-chrome.list.save
google.list
google.list.save
sagemath-monolithic-precise.list
sagemath-monolithic-precise.list.save

The files google-chrome.list and google.list were good candidates for containing duplicates. So checking the contents of both files was required

Output of cat /etc/apt/sources.list.d/google-chrome.list .

deb http://dl.google.com/linux/chrome/deb/ stable main

and of cat /etc/apt/sources.list.d/google.list

deb http://dl.google.com/linux/chrome/deb/ stable main

So there were duplicate entries in the sources of apt Since google-chrome.list only contain a single apt line and it was also listed in google.list file we can safely remove this file with the command

sudo rm /etc/apt/sources.list.d/google-chrome.list

Then the problem was solved

Solution:

Update to deal with sources.list.d dir's files

Example case:

Update to deal with `sources.list.d` dir's files