Great question, PoS actually has stronger guarantees against double-spending than PoW does. For PoW you just need to temporarily rent enough hashpower to do a mild reorg.

PoS has a concept called "block finality", where once a block has been marked finalized it cannot be reorged without committing to getting slashed for 1/3 of total staked ether (several billion dollars). Blocks typically get finalized after 6 minutes. This is possible because you can explicitly check whether validators have voted (attested) for two separate blocks at the same chain height.