在使用弱事件管理器时，如何确定目标是否正在侦听源

I am using the weak event manager in my WPF app to hook up a source of an event to a target, using the XXXEventManager.AddListener(source, this); call. However, I have noticed that my code sometimes hooks up the same source and target more than once. This looks like it causes multiple events to be raised / handled that are effectively the same (i.e. one for each hook up). Is it possible to determine if the target is already hooked up to the event source by interrogating the WeakEventManager (or adding functionality to it) or will I have to implement my own list on the target?I am using the weak event manager in my WPF app