title: Big query using two UNIQUE statements in query
body: <p>My query looks something like this:  </p>
  
  <pre><code>SELECT
    id,
    unique(a) AS un_a,
    unique(b) AS un_b,
  FROM x.y
  
  GROUP BY id
  </code></pre>
  
  <p>I want unique values for a (un_a) and unique values for b (un_b).<br>
  This errs as follows:  </p>
  
  <pre><code>Cannot output multiple independently repeated fields at the same time. Found un_a and un_b
  </code></pre>
  
  <p>Running this query with only one UNIQUE statement works as intended.  What's going on here?  How do I fix it?</p>