Return the duplicated elements of an enumerable (e.g. array) in Ruby

report_duplicate.rb

# Source: http://www.dzone.com/snippets/identify-duplicates-array
module Enumerable
  
  # Returns any duplicates entry if found
  # Usage: <enumerable>.report_duplicate
  def report_duplicate
    inject({}) {|h,v| h[v]=h[v].to_i+1; h}.reject{|k,v| v==1}.keys
  end
end

# Example call:
# an_array = [1,3,5,5,6,7,9,10,14,18,22,22,4,4,4,3,6,13,22,14]
# puts an_array.dups.inspect
# => [3, 5, 6, 7, 14, 22, 4]