When set to True, indicates that this Alarm Source can be acknowledged when active and when inactive, that is, can be doubly acknowledged. When set to False, indicates that this Alarm Source can only be acknowledged once and, when acknowledged, it leaves the Alarm's list. Alarms that do not need acknowledgment (by using the AckRequired property) does not allow that type of customization. Applications earlier than version 2.5 have this property set to False.