Thanks so much guys. really do not know Double Spend can do this. but really such feature does not make good to me.
replace by fee feature is Ok and helps when you want to accelerate confirmation. this double spend..whats the benefit ? only good for cheating or ?
It's actually the Replace by fee exploit that is used to carry out such a type of double spending. Minus their unconfirmed transactions being RBF enabled, the cheat wouldn't have been to broadcast a new transaction with higher fees.