What happens if a miner confirms their own double spend?
Every single Bitcoin node in the network will reject and ignore the block as invalid if there is a double spend in it. The block won't even spread very far, because the peers that first receive it will ban the miner who sent it and not forward on the block to any other nodes.

It is not a big disruption as the check for this is fast. Other mining pools don't need to "remine" it , they'll just recognize the submitted block as invalid, and keep working on their own block.