[1,2,3].permutation(2).to_a が [[2, 2], [2, 3]] になります。

% ./ruby -ve 'p [1,2,3].permutation(2).to_a'  
ruby 1.9.0 (2007-10-09 patchlevel 0) [i686-linux]
[[2, 2], [2, 3]]
% ./ruby test/ruby/test_array.rb             
Loaded suite test/ruby/test_array
Started
....................................................F..........................
Finished in 0.065778 seconds.

  1) Failure:
test_permutation(TestArray) [test/ruby/test_array.rb:1205]:
<[[1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2]]> expected but was
<[[2, 2], [2, 3]]>.

79 tests, 566 assertions, 1 failures, 0 errors
-- 
[田中 哲][たなか あきら][Tanaka Akira]