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How can I programmatically find these duplicate records?
ResponseSets.all.each do |set|
  answer_ids = []
  set.responses.each do |r|
    puts "Response #{r.id}: set=#{set.id}, answer=#{r.answer_id}"
    if answer_ids.include? r.answer_id
      puts "Duplicate found!"
      # r.destroy  # Uncomment when you feel it is safe
    else
      answer_ids << r.answer_id
    end
  end
end

select rs.id, r.answer_id, count(r.id)
from response_sets rs
join responses r on r.response_set_id = rs.id
group by rs.id, r.answer_id
having count(r.id) > 1;

select r.id
from responses r
join (
  select rs.id as rs_id, r.answer_id as a_id, r.id as r_id
  from response_sets rs
  join responses r on r.response_set_id = rs.id
  group by rs.id, r.answer_id
  having count(r.id) > 1
) on r.response_set_id = rs_id
where r.answer_id = a_id and r.id != r_id;