code: func IsDup(err error) bool {
  	// Besides being handy, helps with MongoDB bugs SERVER-7164 and SERVER-11493.
  	// What follows makes me sad. Hopefully conventions will be more clear over time.
  	switch e := err.(type) {
  	case *LastError:
  		return e.Code == 11000 || e.Code == 11001 || e.Code == 12582 || e.Code == 16460 && strings.Contains(e.Err, " E11000 ")
  	case *QueryError:
  		return e.Code == 11000 || e.Code == 11001 || e.Code == 12582
  	}
  	return false
  }
docs: // IsDup returns whether err informs of a duplicate key error because
  // a primary key index or a secondary unique index already has an entry
  // with the given value.