When user is trying to migrate from X to 8.4 or later using existing schema which was imported from an Oracle `imp` tool, migration fails to finish with error:
“ORA-00955: name is already used by an existing object”
This error breaks possibility to restore backup for Oracle DB users.
Main cause of an error is because Oracle DB does not delete indexes of primary keys which were not created during constraint creation.
Schema imported by tools such as “imp” does such thing: it creates index and then constraint, that means that during removal of a constraint, index is not deleted, hence error occur.