duplicate module mix-in depending on order

module M
end
class A
end
class B < A
end
b = B.new
class << b
  include M
end
class B < A
  include M
end
class << b
  p ancestors
end
#[#<Class:#<B:0x007fb7ea8355c8>>, M, B, M, A, Object, Kernel, BasicObject]

#but if class B < A; include M; end before class << b; include M; end
#[#<Class:#<B:0x007ff6e2051438>>, B, M, A, Object, Kernel, BasicObject]