title: neo4j append unique to collection
body: <p>What's the best way to append an element to a collection, and to ensure that element appears only once in the collection?</p>
  
  <p>Basically I have a list of tags and want to append new tags. Something like the query below, except all the element in n.index has to be unique</p>
  
  <pre><code>MATCH n, tag:Tag 
  WHERE n.id='22cfb053-f772-4a3a-83c4-bb733e3dac0a' AND tag.name='hello world' 
  SET n.index=n.index+tag.index 
  RETURN n;
  </code></pre>