title: MongoEngine 0.8.3 NotUniqueError on _id field
body: <p>After upgrading MongoEngine from 0.7.9 to 0.8.3, any attempts to save any existing documents in any collection results in a NotUniqueError (<code>user</code> collection shown in example):</p>
  
  <pre><code>Tried to save duplicate unique keys (E11000 duplicate key error index: foo.user.$_id_ dup key: { : ObjectId('xxxxxx') })
  </code></pre>
  
  <p>I get the same error if I create a new document and save it more than once:</p>
  
  <pre><code>a = Foo()
  a.save()
  a.save() # results in duplicate error
  </code></pre>
  
  <p>Mongo by default creates an index on <code>_id</code> which cannot be removed, and I have no other indexes which use <code>_id</code>. Most issues similar to this that I've seen have been on duplicate indexes that aren't <code>_id</code> and can be removed, but this is really odd. I am doing nothing weird with the <code>_id</code> field, just letting Mongo generate it on its own.</p>
  
  <p>Any ideas on what might be causing this to happen?</p>
  
  <p>Thanks!</p>