title: Mongorestore to update records if already exists without --drop
body: <p>I have a huge dump of collection of data which i have to transfer to another machine every weekend. So i'm planning for an incremental backup and restore. As experimented, mongorestore never merges data if _id already exists.
  Based on the above problem i tried using mongoimport and export with but the same problem exists as the existing records are not merged. Any possible solution would be helpfull.</p>
  
  <blockquote>
    <p>error in mongoimport command caused by :: 11000 E11000 duplicate key
    error index: news.news_data.$<em>id</em>  dup key: { :
    ObjectId('5404410d9f5323ef734dac68') }</p>
  </blockquote>