title: Remove duplicate documents based on field
body: <p>I've seen a number of solutions on this, however they are all for Mongo v2 and are not suitable for V3.</p>
  
  <p>My document looks like this:</p>
  
  <pre><code>    { 
      "_id" : ObjectId("582c98667d81e1d0270cb3e9"), 
      "asin" : "B01MTKPJT1", 
      "url" : "https://www.amazon.com/Trump-President-Presidential-Victory-T-Shirt/dp/B01MTKPJT1%3FSubscriptionId%3DAKIAIVCW62S7NTZ2U2AQ%26tag%3Dselfbalancingscooters-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D165953%26creativeASIN%3DB01MTKPJT1", 
      "image" : "http://ecx.images-amazon.com/images/I/41RvN8ud6UL.jpg", 
      "salesRank" : NumberInt(442137), 
      "title" : "Trump Wins 45th President Presidential Victory T-Shirt", 
      "brand" : "\"Getting Political On Me\"", 
      "favourite" : false, 
      "createdAt" : ISODate("2016-11-16T17:33:26.763+0000"), 
      "updatedAt" : ISODate("2016-11-16T17:33:26.763+0000")
  }
  </code></pre>
  
  <p>and my collection contains around 500k documents.  I want to remove all duplicate documents (except for 1) where the ASIN is the same</p>
  
  <p>How can I achieve this?</p>