Ruby — Open class and define method when only original method is not defined

Just one idea. Is it a bad practice?

class A
def some_method1
puts 'Naked method!(1)'
end
def some_method2
puts 'Naked method!(2)'
end
end
class A
def some_method1
puts 'Overrided method!(1)'
end
unless A.instance_methods(false).include?(:some_method2)
def some_method2
puts 'Overrided method!(2)'
end
end
end
A.new.some_method1 # Overrided method!(1)
A.new.some_method2 # Naked method!(2)