## Code

class Array
  def dups
    m = []
!!    self.inject(Hash.new(0)) {|c,x|
!!      c[x] += 1
!!      m << x if c[x] > 1
!!      c
!!    }
    m
  end
end

a = [1,2,3,4,4,5,2]
puts a.dups.inspect

## Result

[2, 4]