uniq_with_block.rb

Array.class_eval do
  def uniq_by(&block)
    self & Hash[
      *inject([]) do |memo, item|
        memo << block.call(item) << item
      end
    ].values
  end
end

puts ['c', 'a', ['Aa'], ['C', ''], 'b', 'A'].uniq_by { |s| s.to_s.downcase }.inspect
# => [["Aa"], ["C", ""], "b", "A"]

Something like [['C', ''], 'a', ['A'], ['c'], 'C', 'b', 'A'].uniq_by { |s| s.to_s.downcase } doesn't work yet (odd number of arguments for Hash).

And now it works.