Hi,
It's probably that the Ideal payment gateway sends many payment notifications instead of one.
And the idealcheckout plugin doesn't handle that properly.
You will have to add the line:
if($dbOrder->order_status == $order->order_status) return true;
in the onPaymentNotification function of your plugin.
Probably after that:
$order->order_status = $element->payment_params->verified_status;
However, I would recommend you to contact the developers of that plugin so that they can confirm the fix and potentially include it in their plugin as we are not the developers of that plugin.