Extracts the indexes of the duplicate elements including the original element

duplicate_index.rb

def duplicate_index(array)
  size = array.size
  duplicate = []
  duplicate_indexes = []
  array.uniq.each do |a|
    duplicate_index = []
    size.times do |i|
      duplicate_index << i if array[i] == a
    end
    duplicate << duplicate_index if duplicate_index.size > 1
  end
  duplicate
end