Order-related / Delivery-related billing of item category TAN

Hello SD guys,

when I use sales order with item category TAN, with Billing Relevance parameter = A (delivery-related billing)

i generate my delivery, then create the billing document

then, if I try to generate DOUBLE invoice from sales order for the same item, the system ALLOW me ! which is not normal

could you please explain this behavior

thanks

Sameh