Spurious "W: You may want to run apt-get update to correct these problems" when "Duplicate sources.list entry" is only warning

[Scott Ritchie]

If you create duplicate lines in sources.list, and then run apt-get update, you receive something like this:

W: Duplicate sources.list entry http://security.ubuntu.com/ubuntu/ precise-security/multiverse amd64 Packages (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_precise-security_multiverse_binary-amd64_Packages)
W: Duplicate sources.list entry http://security.ubuntu.com/ubuntu/ precise-security/multiverse i386 Packages (/var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_precise-security_multiverse_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

The final warning is not helpful.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Vlad Orlov (monsta)]

Yes, this warning is totally useless because apt-get update won't magically remove the duplicates.

Actually, apt is full of strange messages that are completely unclear to the users (like "you have held broken packages" when nothing is actually being held). This is just one of them.