title: Remove duplicates after $sample
body: <p>I'm using <code>$sample</code> to get some random documents from a collection:</p>
  
  <pre><code>db.myCollection.aggregate(
     [ { $sample: { size: 100 } } ]
  )
  </code></pre>
  
  <p>Due to the nature of <code>$sample</code>, same documents may get retrieved more than once, so my sample of 100 may contain duplicates. How can I remove the duplicates after sampling?</p>