I ran into this problem when upgrading from 0.7.12 to 1.4.9.
The index gets created during one of the upgrade steps so I could not
manually drop it.

My workaround was to ignore errors when creating the index in
add_class_key_required_unique_constraint() in rdbms_common.py:

        try:
            self.sql(sql)
        except Exception:
            pass