lncli bumpfee is lame. It cannot RBF the original transaction, it can only CPFP if there is a change output going back to your LND wallet. It can RBF that second transaction if the fee was still insufficient. This is because it was introduced for bumping fee of channel opening/closing transactions, which are multisig and cannot be RBF'd by a single peer.

For a primer how to evict the low fee tx and double spend UTXOs read this: https://medium.com/@goryachev/bitcoin-node-security-case-study-e1ca00a378b5