These days, I am a big proponent of end-to-end: making sure your core pieces are not trying to too smart, because being too smart will actually prevent new packages from innovating freely on top of your core.
In this case, the piece of code that is best going to be able to tell if there is a potential overlap of notifications is the bboard package itself. The underlying notifications package shouldn't try to figure this out for every package that uses it....
But I see your point, and eventually bboard should be smart about this.
