This error means you're trying to register more than one order with the same "merchant_order_id" value, the value of the key "merchant_order_id" should be unique per order.

So in this case, first try to delete the key "merchant_order_id" from your order request, if the order successfully registered, please check the assigned value of the "merchant_order_id" key and make sure that you're sending a unique value for this key with each order registration request.