This is our instructor's version of the validator, using .persisted? to check that the current record has already been saved and then taking it out of the 'duplicates_responses' set.

Validator-fixed-2

def respondent_has_not_already_answered_question
  duplicate_responses = self
    .question
    .responses
    .where("responses.respondent_id = ?", self.respondent_id)

  if persisted?
    duplicate_responses =
      duplicate_responses.where("responses.id != ?", self.id)
  end
    
  unless duplicate_responses.empty?
    errors[:respondent_id] << "cannot vote twice for question"
  end
end