You can use the add_post_meta and update_post_meta filters to accomplish this. If you return anything other than null in these filters the post meta will not be saved.
add_filter('add_post_metadata', 'check_metadata', 10, 5);
function check_metadata($check, $object_id, $meta_key, $meta_value, $prev_value){
/*** Check if the metadata exists via sql ***/
if($exists) {
/*** Notify you or something ***/
return true;
else {
return;
}
}