title: Cannot build a unique index on a column with no duplicate values
body: <p>First I ran </p>
  
  <pre><code>select beatid, count(*)
  from beat
  group by beatid
  having count(*) &gt; 1
  </code></pre>
  
  <p>And I got no rows as the result.</p>
  
  <p>So then I ran</p>
  
  <pre><code>CREATE UNIQUE INDEX BEAT_IDX_UNQ ON BEAT (BEATID);
  </code></pre>
  
  <p>and got</p>
  
  <blockquote>
    <p>Invalid insert or update value(s): object columns are constrained - no
    2 table rows can have duplicate column values. attempt to store
    duplicate value (visible to active transactions) in unique index
    "BEAT_IDX_UNQ".</p>
  </blockquote>
  
  <p>What's up with that?</p>