CAY-1794 Duplicate attributes in discriminator columns of PersistentDescriptor

(cherry picked from commit 0014b8c15ddf7a71ae5b8bdb3e82909191c49c8b)

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1446648 13f79535-47bb-0310-9956-ffa450edef68