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I am using entity url reference to create nodes of content "b" with reference to content "a".
What I need to do is limit users to create only 1 content of content "b" per content "a" per user. So
User A creates content B reference Content A. The user can no longer reference that node.
Limit Each user to only reference content a one time. They can reference other nodes in content A with content B, but only reference each node once.