gistfile1.rb

class A
  class << self
    def foo
      puts "foo"
    end
  end
end

A.foo

class A
  class << self
    alias original_foo foo
    def foo
      puts "I say bar but only once.."
      alias foo original_foo
    end
  end
end

A.foo
A.foo

# => foo
# => I say bar but only once..
# => foo