Delinquency notice letter setup

I am trying to set up the Customer Delinquency Notice letters, P03B250, and cannot find how to set the notifications so that, if there is an invoice which has previously had a notification letter and another invoice becomes due for a first notice letter (same customer), both the invoices are not printed on the second reminder. Currently, no first reminder is issued for the invoice that has only just become due but both are printed on the second reminder.

So, for example, this is my issue:

Invoice 1 became overdue on 30th April and notice 1 was printed. OK.

There are 7 days between notifications and the processing options are set for severity levels.