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.
0-conf transactions have timestamps and the receiving wallet/node checks for the tree of transactions to even allow the spending of unconfirmed transactions up to a limit of 25 for now, there is on-going research and testing to up this limit to 500 that was sponsored by SatoshiDice https://twitter.com/PeterRizun/status/1181980303033692162
For a list of transactions trying to double spend BCH and failing at it, see this https://doublespend.cash/
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.