From my quite limited understanding, RBF basically allows to cancel a transaction (since you can set the fee to something that would never be accepted in a block) which is even worse than a regular double spending, because in the end, nothing at all is spent.

But even without RBF, there's nothing stopping you from spending the same coins online and in a restaurant at the same time. I'm not even sure if a restaurant would know that you and you're friend aren't actually spending the same money twice for your respective meals.