title: Can not insert unique document using gmongo in Grails
body: <p>I using gmongo (for Grails) library to insert data into mongodb. I want to insert with unique element. 
  I try create a Unique Index for MongoDB:</p>
  
  <pre><code>db.collection.ensureIndex( { "object_id": 1 }, { unique: true } )
  </code></pre>
  
  <p>However, There are some duplicated elements when I try to insert many time one data through webservice at the same time. Maybe, I got 3-5 duplicated elements when I insert 500 documents</p>
  
  <p>Thank in advance.</p>