Package: apt
This problem was discovered because of the following situation. An Ubuntu user had added a ppa with a backported lib in it. He then installed the lib. He later added another ppa, unrelated to the first, which also had the same lib, backported, with the same version string. He tried to install the muliarch version of the lib, and apt selected the second ppa for that. But the changelogs were different. Apt refused to overwrite the changelog from ppa 1 with the changelog from ppa 2. The user's system was then left in a broken state with -f install not being able to fix it. I discovered that apt chooses the preferred repository when all else is equal using essentially whichever entry appears first in the alphabet. Obviously I think this can improve. Apt could ask the user for a decision on which repo to use, or could simply use the repo the original lib was taken from to avoid this kind of essentially meaningless conflict in the future, or offer the user an easier way out of it (all of this happened because of a few differences in the changelog, hardly important enough reason to break a system).
Here's the user's apt-cache policy for that package:
Here's how the result left his system: