Thanks Peter.

I think you’re right either the block validation needs to be put on hold until avalanche finishes or as an alternative the miners can probably do the same trick with SPV mining where they conditionally mine on top of it and then revert if the block turns out to be invalid. The decision to conditionally mine on it or not could be based on the node’s estimation of how likely avalanche will say the tx in the block is invalid.

This seems like this would only happen if a double spend was received around the same time a block was created are it caused a race. Otherwise miners would not intentionally be adding unannounced transactions.