Rails 2.1 eager loading doubleup bug
I've created Rails lighthouse ticket #1110 for another bug in the new 2.1.x association preloading code that was causing some has_manys to get loaded with two copies of each record in the collection (and less seriously, double SQL loads of all types of collections).
The problem turned out to be triggered by associations that have :includes defined on them themselves when the :includes are also supplied in the find call. My patch is on the ticket. +1s welcome :).