Hydrating mid-stream renders a nested Errored's fallback twice #4374

Found by the async app generator's hydration phase (#4371). Reproduces on next 5f60713.

function App() {
  const a = createMemo(async () => {
    await sleep(10);
    return 1;
  });
  const b = createMemo(() => {
    if (a() === 1) throw new Error("boom");
    return a();
  });
  return (
    <main>
      <Errored fallback={e => "[ROOT]"}>
        <Loading fallback="[LOAD]">
          <Errored fallback={e => "[OUTER]"}>
            <Errored fallback={e => "[INNER]"}>
              <span>{b()}</span>
            </Errored>
          </Errored>
        </Loading>
      </Errored>
    </main>
  );
}

The server streams the Loading's fragment as <!--!$-->[INNER], plus a's value and b's error.

• Hydrate after the chunk arrives: <main>[INNER]</main>, which is correct.
• Hydrate the shell, then apply the chunk (the client boots mid-stream): <main>[INNER][INNER]</main>. The fallback is inserted twice. The duplicate doesn't go away on later updates, and a sibling after the boundaries keeps its place after both copies.

Without the outer Erroreds (a single Errored directly in the Loading, no root Errored), both orders show [INNER] once.