“duplicate sources” error message during an Ubuntu Live CD session

Steps to reproduce:
1) boot into live cd
2) click 'Try Ubuntu'
3) launch terminal (e.g. click big fat button -> search for 'Terminal')
4) type $ sudo apt-get update
5) see errors:

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

Expected:
5) don't see any Duplicate sources.list errors.

Same with quantal daily live 2012-06-19