InfoPath Dev

I have a shipping form that is used for international shipping.  When the form opens it autofills the shipping invoice number by adding 1 to the last invoice number in a SharePoint list.  The problem is that if multiple copies of the form are open at the same time they all get submitted with the same invoice number.  Is there a way to verify on the initial submission of the form that the invoice number is a valid number based on the list?

At submit time, you can have rules fire so that it does a check against your list to see if there is an existing invoice number.  You'll need a secondary data connection to the list and then do a query on the invoice number to see if it returns any results.

Another option is that you could also create the invoice number on submit to ensure that it will always succeed.