Dunning Callback tasks

Does anyone know what is supposed to happen to Dunning Callback tasks if a new Dunning process is run and a new callback task is created?  Do the old tasks automatically close when a new Callback task is created or do they remain open?

For example, you run Dunnings January 5, by January 10 you have received no payment so a callback task is created.  This task remains open and on February 5, you run another Dunning process.  On February 10 no payment is received and a new callback task is created.  What happens to the callback task created on January 10?

Are there any reports that identify what Callback tasks are open?