report_duplicate_elements.rb

# Takes an array (enumerable?) as input
# Return an array containing elements occurring multiple times in input
def report_duplicate_elements(list)
  occurs_multiple_times = []
  already_seen = []

  list.each do |element|
    if already_seen.include?(element)
      occurs_multiple_times << element
    else
      already_seen << element
    end
  end
 
  # Discard dups from elements occurring more that twice
  occurs_multiple_times.uniq
end

Nice idea!