code: func (q *Query) validateArgField(elem *callStackElem) {
  	if _, exists := elem.call.Args[elem.lastField]; exists {
  		panic(fmt.Sprintf("%s: %s", duplicateArgErrorMessage, elem.lastField))
  	}
  }
docs: // validateArgField ensures that field does not already
  // exist as a key in the Args map before adding the new
  // key/value.