Thanx for coming back to me so quickly. If I have time today I will try and produce a small sample as this forms part of a much larger app. To recreate you would need to follow the below steps:
1. Add a Prism module to a Prism shell application project that includes a single view. The view is marked with the [RegionMemberLifetime(KeepAlive = false)] attribute and is injected with the interface of a simple view model
2. In the simple view model expose an IInteractionRequest property and set this up to use the basic Notification class to raise against
3. In the view's XAML add the Interaction.Triggers element as above and simply omit the child window which will use the NotificationChildWindow by default.
4. In the module init of the new module ensure the registered view model is set up as long-lived i.e. ContainerControlledLifetimeManager
5. In the view add a button that raises the interaction request in the view model
6. Run the application and navigate to the view - clicking the button will display a notification window
7. Navigate to a different module and then back again - click the button again and this time you should recieve 2 notification windows on top of each other