Joomla! 1.0.12 Cannot redeclare error in admin section

This article is incomplete and was first written in June 2007
for the BeezNest technical website (http://glasnost.beeznest.org/articles/360).
This article is meant to help people who run into the same problem as I did, and hopefully will help me next time I have to deal with the same problem as well.

The problem appearance

Now OK, this was a peculiar situation, but I had to move a Joomla! 1.0.12 installation (which had just been migrated from Mambo to Joomla!