Scheduled Proceses for Dunning Letters

I am currently setting up dunning in our development environment. I have been able to complete the configuration and I am able to send dunning letters. I just have one question on the scheduling of these processes.

I am trying to setup the aged dunning process to run automatically, but I do not want it to duplicate letters to customers. I thought it would send a letter during the soft period, then not send another letter until the customer was in the medium period and so on.

For example:

1-30 Days - Soft letter sent to customer.

31-60 Days - Medium letter sent to customer, on the 31st day.