composer require league/flysystem-replicate-adapter
$source = new League\Flysystem\Adapter\AwsS3(...);
$replica = new League\Flysystem\Adapter\Local(...);
$adapter = new League\Flysystem\Replicate\ReplicateAdapter($source, $replica);
What's cool about this is that you can chain them to replicate to more then 1 other storage system.
$adapter = new League\Flysystem\Replicate\ReplicateAdapter($source, $replica);
$anotherReplica = new League\Flysystem\Adapter\Dropbox(...);
$adapter = new League\Flysystem\Replicate\ReplicateAdapter($adapter, $anotherReplica);