Don't confuse RBF with double-spend! That's totally not the same thing. Double spending is an attack scenario where you spend the same funds twice (like, to 2 different addresses). RBF just allows you to bump the fee to have the transaction be verified quicker.
Actually RBF is like a sub-category of double spend.
Technically speaking when the same UTXO is spent in 2 different transactions, we call that a double spend. When you bump the fee of a transaction you are actually creating an entirely different transaction (it has to even be signed again) and the fact that the two transactions differ only in the output amount doesn't change that.
What RBF does is that it "reserves the right to double spend" the tx marked by RBF for the signer but with certain restrictions, such as the increased fee has to be proportional to the tx size not any small amount.