Hi,
The attribute post_id and other attributes of the form (form settings) are saved to the database in the table wp_options with the key mbfs_key. Each form has its own key based on the form settings.
So it is possible to create a new form and set the post_id to 0 and generate a new key to create a new post after submitting but you will need a lot of custom code to do that.
Instead of modifying the post_id attribute of the form, you can try to use the action hook rwmb_frontend_before_save_post and set the post_id to 0 to duplicate a post. For example:
add_action( 'rwmb_frontend_before_save_post', function( $object ) {
$object->post_id = 0;
} );
.
