Seeking spec clarification: repeated text in the same live region

I'm seeing different behaviors in different implementations, when I set the
innerText of a live region to the same text it was before.
- Firefox: the text repeats. I remember coding it to repeat on purpose back
when I worked on Firefox accessibility.
- Safari and Chrome: the text does not repeat

What is the correct behavior?

Here's my use case for repeating the text. Imagine an alert. The alert
might say something like "This passwords do not match" or something else as
a response to validation of user input. Every time the alert is updated,
there is a visual animation showing something new happened. However, just
because the text happens to be the same as what it was 5 seconds ago, this
time it is not spoken. This may be a contrived example.

Looking for feedback!

Thanks,
Aaron