title: duplicate multi column entries postgresql
body: <p>I have a bunch of data in a postgresql database. I think that two keys should form a unique pair,
  so want to enforce that in the database. I try</p>
  
  <pre><code>create unique index key1_key2_idx on table(key1,key2)
  </code></pre>
  
  <p>but that fails, telling me that I have duplicate entries.</p>
  
  <p>How do I find these duplicate entries so I can delete them?</p>