"identical" alerts should not lead to revoking the old alert

[berthubert]

As a simple example, dailyChime issues a new chime every day. This leads to a notification that the old chime is over. And then there is the new one. This is silly. We should have an identifier that says this is still the same alert, and that there is no need to announce that the old one was over.

This could happen with an explicit alert key perhaps.