Create Reminders Routine - Shows Same Invoice TwicePosted

When I am running the create reminders routine on NAV 2009 SP1 with the ‘Only Entries with Overdue Amounts’ unticked it correctly brings through my invoices outstanding but not due, which is what I want, but it is showing the same invoice twice. All Invoices which are actually due only show once.

The Invoice is due on the 30/09/2011 and I am running the Reminder Letters Routine with a Document Date of 01/08/2011 and the Use Header Level Selected.

Has anyone come across this issue before and how can I prevent this Invoices appearing twice?