ShellNavigationManager: disallow multiple navigations at the same time

[Dreamescaper]

Sometimes multiple navigations might be triggered at the same time - e.g. if you tap some element multiple times (fast), or tap multiple elements at the same time (multitouch).
Currently it results in same page being added to the navigation stack twice.

Of course, it can be handled in user code. Still, would it make sense to disable multiple navigations at the same time at all on MBB side? Is there any downside in that?