dup/clone method has an unexpected behavior when an 'id' field is present in the document

I have a problem duplicating documents. Some of my mongodb documents are containing a field ``id`` different from the real identifier ``_id``. 

```{"_id" : ObjectId("4f243f6910d344d6400001cf"), "score" : 1.2, "id" : 290421 }```
	
When I duplicate documents with both ``_id`` and ``id`` fields the duplicated record is saved with the value of the ``id`` in the ``_id`` field because of the aliasing resulting in: 

```{"_id" : 290421, "score" : 1.2 }```

I don't know if the solution in this pull request is general enough, but it seems to make sense with the created aliasing.

Sorry about the missing test but I didn't know how exactly create a test case for it.