title: Postgres select duplicate on one column but different on another
body: <p>I have a database with two columns:</p>
  
  <p><code>author_id, message</code></p>
  
  <p>And entries like:</p>
  
  <pre><code>123, "message!"
  123, "message!"
  123, "different message"
  124, "message!"
  </code></pre>
  
  <p>I want to do a query that allows me to select either:</p>
  
  <p><code>123, "message!"</code></p>
  
  <p>or </p>
  
  <p><code>124, "message!"</code></p>
  
  <p>Essentially, entries where the <code>message</code> is the same, but the <code>author_id</code> is different.</p>
  
  <p>I then want to delete one of these entries. (it doesn't matter which one, just that I can select only one of them).</p>
  
  <p><a href="https://stackoverflow.com/questions/21669202/find-rows-with-multiple-duplicate-fields-with-active-record-rails-postgres">This question</a> gets me close but it's for duplicates across two columns.</p>