title: PyMongo throwing "E11000 duplicate key error index"
body: <p>Duplicate documents are expected to be inserted into the mongodb collection, so an index was created with <code>unique=True</code> and <code>dropDups=True</code>.</p>
  
  <pre><code>db.myCollection.create_index("timestamp", unique=True, dropDups=True)
  </code></pre>
  
  <p>However if the same set of documents are inserted twice, the first insert goes fine but the second insert throws the error</p>
  
  <pre><code>db.myCollection.insert(json.loads(df.to_json()).values())
  </code></pre>
  
  <p><code>DuplicateKeyError: E11000 duplicate key error index: myDb.myCollection.$timestamp_1  dup key: { : 1385290560000000000 }</code></p>
  
  <p>I am confused as to why <code>dropDups=True</code> is not working.</p>