Using the C++ driver, nothing prevents a user from inserting a document that has two fields which share a name. I don't believe that the behavior of future updates on this document is documented or well-defined (as a sidenote, operating on this document can't crash the server, right?).

See:

> db.test.find()
{ "_id" : ObjectId("5058f25f7d54ac9bc09d4e57"), "a" : 0, "a" : 0 }
>

as a product of:

#include "mongo/client/dbclient.h"
using namespace mongo;
int main() {
  DBClientConnection c;
  c.connect("localhost");
  BSONObj bsonData = BSON("a" << 0 << "a" << 0);
  c.insert("test.test", bsonData);
  return 0;
}