title: Duplicate documents on _id (in mongo)
body: <p>I have a sharded mongo collection, with over 1.5 mil documents. I use the _id column as a shard key, and the values in this column are integers (rather than ObjectIds).</p>
  
  <p>I do a lot of write operations on this collection, using the Perl driver (insert, update, remove, save) and mongoimport.</p>
  
  <p>My problem is that somehow, I have duplicate documents on the same _id. From what I've read, this shouldn't be possible.</p>
  
  <p>I've removed the duplicates, but others still appear.</p>
  
  <p>Do you have any ideas where could they come from, or what should I start looking at?
  (Also, I've tried to replicate this on a smaller, test collection, but no duplicates are inserted, no matter what write operation I perform).</p>