list.rb

    def shuffled_users
      shuffled = users.sort_by { rand }
      while shuffled == users
        shuffled = users.sort_by { rand }
      end

      shuffled
    end

list_spec.rb

  describe "#shuffled_users" do
    subject { described_class.new(users) }

    it 'should return users in random order' do
      rand_sequence = [2, 1, 3]
      subject.should_receive(:rand).and_return(*rand_sequence)

      subject.shuffled_users.should_not == subject.users
    end

    it 'should continue shuffling' do
      first_run = [1, 2, 3]
      rand_sequence = first_run += [2, 1, 3]
      subject.should_receive(:rand).and_return(*rand_sequence)

      subject.shuffled_users.should_not == subject.users
    end
  end