- Fixed bug in single table inheritance where a chain of joins

- Fixed bug in single table inheritance where a chain of joins
that included the same single inh entity more than once
(normally this should raise an error) could, in some cases
depending on what was being joined "from", implicitly alias the
second case of the single inh entity, producing
a query that "worked".   But as this implicit aliasing is not
intended in the case of single table inheritance, it didn't
really "work" fully and was very misleading, since it wouldn't
always appear.
fixes #3233