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.