Why do I receive the "duplicate sources" error message during an Ubuntu 12.04 Live CD session?

Question

When I boot the Ubuntu 12.04 desktop Live CD and run the command sudo apt-get update, the final messages are the warning messages below.

Reading package lists... Done
W: Duplicate sources.list entry cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)/ precise/main i386 Packages (/var/lib/apt/lists/Ubuntu%2012.04%20LTS%20%5fPrecise%20Pangolin%5f%20-%20Release%20amd64%20(20120425)_dists_precise_main_binary-i386_Packages)
W: Duplicate sources.list entry cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)/ precise/restricted i386 Packages (/var/lib/apt/lists/Ubuntu%2012.04%20LTS%20%5fPrecise%20Pangolin%5f%20-%20Release%20amd64%20(20120425)_dists_precise_restricted_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

However, there do not appear to be any duplicate sources in /etc/apt/sources.list

# /etc/apt/sources.list

deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ dists/precise/main/binary-i386/
deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ dists/precise/restricted/binary-i386/
deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ precise main restricted
deb http://archive.ubuntu.com/ubuntu/ precise main restricted
deb http://security.ubuntu.com/ubuntu/ precise-security main restricted
deb http://archive.ubuntu.com/ubuntu/ precise-updates main restricted

I also ran the command gksudo software-properties-gtk and noticed that there are two entries under the Other Software tab for Cdrom with Ubuntu 12.04 'Precise Pangolin'

Removing these two entries in Other Software also eliminates the Duplicate sources.list entry warning messages from apt-get update (as well as other apt-get functions).

I do not understand why those two entries for Cdrom with Ubuntu are present in the Other Software tab. The CD is also already selected as a possible install source under the Ubuntu Software tab. That should be enough, shouldn't it?

Is there an error in the Ubuntu 12.04 Live environment's default configuration of Software Sounces? Or am I doing something (else) wrong?

Is it OK to remove the Cdrom entries under Other Software to eliminate the W: Duplicate sources.list entry cdrom: message?

---

Update:

I forgot to look at /etc/apt/sources.list after removing the entries from the Other Software tab. After doing that its contents becomes:

# /etc/apt/sources.list

deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)]/ precise main restricted
deb http://archive.ubuntu.com/ubuntu/ precise main restricted
deb http://security.ubuntu.com/ubuntu/ precise-security main restricted
deb http://archive.ubuntu.com/ubuntu/ precise-updates main restricted

I'm still confused as to what is going on here. Is
dists/precise/main/binary-i386/ and
dists/precise/restricted/binary-i386/ the same thing as
precise main restricted??

I didn't think so when I first looked at it, but then again removing those entries eliminates the Duplicate sources.list warning messages.

Answer 1

What's happening is that the Precise AMD64 internet repositories also include the i386 repositories by default. I assume the reason is so that you can seamlessly install and run i386 programs after simply installing ia32-libs.

It looks like this is also true for the CDROM repos -- that's all there is to it.

Here's what an extremely simple one-line sources.list does on an AMD64 Precise system:

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

• sudo apt-get update output (note Get:5 and Get:6):

  Ign http://archive.ubuntu.com precise InRelease
  Get:1 http://archive.ubuntu.com precise Release.gpg [198 B]
  Get:2 http://archive.ubuntu.com precise Release [49.6 kB]
  Get:3 http://archive.ubuntu.com precise/main amd64 Packages [1,273 kB]
  Get:4 http://archive.ubuntu.com precise/restricted amd64 Packages [8,452 B]                 
  Get:5 http://archive.ubuntu.com precise/main ***i386*** Packages [1,274 kB]                       
  Get:6 http://archive.ubuntu.com precise/restricted ***i386*** Packages [8,431 B]                  
  Get:7 http://archive.ubuntu.com precise/main TranslationIndex [3,706 B]                     
  Get:8 http://archive.ubuntu.com precise/restricted TranslationIndex [2,596 B]               
  Get:9 http://archive.ubuntu.com precise/main Translation-en [726 kB]                        
  Get:10 http://archive.ubuntu.com precise/restricted Translation-en [2,395 B]                     
  

Answer 2

You see this, because it's a bug =) You shouldn't see it. Filed as https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1015495 after discussing with cjwatson.