A status region that reads Saved twice

Save clicked. Old tree has no region. New tree arrives with the region and the text in one go. Same task, same batch of mutations. The screen reader gets a brand new region and its content at the same moment, and what it does with that is up to the reader.

That's why MDN's advice reads like an event loop instruction. Create the region empty, put the content in from a later task, and for regions created from script it literally suggests setTimeout(). Keeping the node mounted gets you the same thing for free, because the region was registered long before the first message.

I can't tell you why it came out twice rather than zero times. From that starting point both are plausible. Your fix removes the question, so I'd stop chasing it.

The region stays mounted now. Save once and it says Saved. Change one field, save again, and the text is Saved again. Same string, same node. From the reader's side nothing changed, so does it get announced at all? And if the trick is to clear it and set it back from a later task, isn't that the same two-step shape that gave tess.p a double read?

There are two questions in there, and React answers the first one before any screen reader gets involved. If the component re-renders Saved as Saved, React sees the same text and leaves the DOM alone. No mutation, nothing for the reader to notice, nothing announced.

The clear-and-set trick is older than every framework in this thread (it was a jQuery-era habit, .text('') and then a setTimeout to put the message back, and people copied it around long after anyone remembered which reader it was for), and it isn't the shape that gave tess.p the double read. Her region arrived with its text already inside. Here the region has been registered the whole time, the empty step is a removal, and with the default aria-relevant of additions text a removal isn't meant to be announced. The second step is an ordinary text change.

Whether a given reader repeats identical text after that dance still varies, so it's worth hearing it on the one you care about before trusting it.