title: Fast way to find duplicates on indexed column in mongodb
body: <p>I have a collection of md5 in mongodb. I'd like to find all duplicates. The md5 column is indexed. Do you know any fast way to do that using map reduce. 
  Or should I just iterate over all records and check for duplicates manually?</p>
  
  <p>My current approach using map reduce iterates over the collection almost twice (assuming that there is very small amount of duplicates):</p>
  
  <pre><code>res = db.files.mapReduce(
      function () {
          emit(this.md5, 1);
      }, 
      function (key, vals) {
          return Array.sum(vals);
      }
  )
  
  db[res.result].find({value: {$gte:1}}).forEach(
  function (obj) {
      out.duplicates.insert(obj)
  });
  </code></pre>