Planned item supply duplicate of entered purchase orders

We run time phased supply plans (all items and assemblies are time phased).  When the planned purchase orders are placed, invariably one or two vendors will give a commit date later that the demand date.  We know how to generate reschedule messages to alert us to the late supply for a given demand.  But we don't want a subsequent supply plan to create another planned purchase order.  I would have figured setting up a reschedule in/out window for each part would cause reschedule messages instead of new planned demand.  But you just get the message in addition to another planned purchase.  I'd like to suppress adding new planned purchases once the PO has been firmed, especially if the demand date falls within the reschedule window.