title: InsertMany throw E11000 duplicate key error but insert successfully
body: <p>I am using mongoose to insert bulk data to database.
  The <code>insertMany</code> throw E11000 duplicate key error although the data is saved succeed in database.</p>
  
  <p>Here is my codes: </p>
  
  <pre><code>  var data = [
        {_id: '598d7ae7c3148a6d853866e8'}, 
        {_id: '598d7afdc3148a6d853866f4'}
      ]
  Movies.insertMany(data, function(error, docs) {});
  </code></pre>
  
  <p>This happen infrequently.</p>
  
  <p>Appreciated any help.</p>
  
  <p>Thanks </p>