Rubyでselfを返すメソッドってあったっけ？ ref: http://qiita.com/items/2524

dupli.rb

class Object
  def tap
    block_given? ? super : self
  end
end

class Array
  def dupli 
    self.group_by(&:tap).reject{ |k, v| v.one? }.keys
  end
end

ary = [*0.step(8, 2)] + [*0..9]   # => [0, 2, 4, 6, 8, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
ary.dupli                         # => [0, 2, 4, 6, 8]

duplicate.rb

class Array
  def duplicate
    self.group_by{ |e| e }.reject{ |k, v| v.one? }.keys
  end
end

ary = [*0.step(8, 2)] + [*0..9]   # => [0, 2, 4, 6, 8, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
ary.duplicate                     # => [0, 2, 4, 6, 8]