Get Job Usage from Sales Invoice

Hi
I have been able to post two invoices for the same client using the same job number. Looks absurd but here is how you can do it.

1. Create a new sales invoice, input your customer code and so on
2. Do the 'Get Job usage' and select a Job No.
That will bring all transactions done on this job on the sales invoice. Dont do anything else with that invoice.
3. Now create a new invoice for the same customer and repeat step 2 with the same job number.
4. Try to post the first invoice.
5. Then try to post the second one as well. Navision does not block the user from posting the second invoice and the customer ends up with 2 invoices with same details.

Is there a fix for this ?

Thanks and regards
Videsh