Method to return items that appear more than once in an array (or Enumerable)

   1  
   2  module Enumerable
   3    def dups
   4      inject({}) {|h,v| h[v]=h[v].to_i+1; h}.reject{|k,v| v==1}.keys
   5    end
   6  end

How it works: the inject method builds up a hash with keys for the unique values in the array and
a value representing the number of times the entry is found in the array. Any entries with a
value of 1 (i.e. not duplicated) are removed, and the resulting keys represents the duplicated
entries in the original array.

Example:

   1  
   2  arr = %w{foo bar baz bar baz qux foo zub}
   3  puts arr.dups.inspect
   4  # => ["baz", "foo", "bar"]