Duplicate joins when combining scopes and :include


I have an app using Rails 2.3.2 that chains together multiple scopes
with joins and conditions on different tables. When I add :include to
the finders that use these scopes, I get an Mysql::Error saying Not
unique table/alias for the table that I’m including, and joining on in

The join strings are exactly the same, but the same left outer join is
repeated in the SQL. I thought them being string identical would
prevent that from happening?

Is this a known problem with any workarounds? At the moment I just
skip the eager loading, which is hurting performance.