If you use 'post_id' => 'new_post', ACF will automatically create a new post using the values form the new_post array. ACF should not be creating a new post if you use just new for the post_id.
Do you have other front end forms that are creating new posts? If you do you’ll need to use a different post_id for each. For example in your above code I would use something like `new_candidates_post’
acf_form(array(
'post_id' => 'new_candidates_post',
'field_groups' => array(7,18),
'submit_value' => 'Book'
));
function my_pre_save_post ($post_id) {
// check if this is to be a new post for candidates
if ($post_id != 'new_candidates_post') {
return $post_id;
}
// Create a new post ....
}
