Sales oder with service product

Hi,

I have question regarding the sales order with service product where service order is created.

I create a sales order with service product. Automatically a service order is created, that's all fine.

When try to bill the sales order, I can just bill this using VF01. After that I try to create a billing request for the service order created, suprisingly I can create the billing request and can bill the billing request. Which means that I can bill the service product twice in this case.

I there any logic here? How can I prevent that the service product is billed twice?

Thanks in advance for your help