Hi @aaron and all of the DEV Team
we found another Bug in Marketpress.php main Plugin File.
line 3916 has do_action( 'mp_order_paid', $order );
when 3915 calls the function update_order_status which also calls the action
so eventually firing the same hook twice
this bug also exists in version 2.8 beta 2
a fast removal of the action call from the function update_order_payment_status will fix it
Please fix it