await race condition - :then block renders twice if the promise is resolved and then changed at the same time

[SystemParadox]

Describe the bug

If you have an await block and resolve the promise and then change it to a new promise at the same time, the :then block renders twice. If you have any components in this block their onMount will be called twice, but if you later remove them the unmount will only be called once.

Reproduction

https://svelte.dev/repl/d3acebb52bf34f73a17d8afa2ddd19ce?version=3.49.0

Some notes:

• The Foo component intentionally errors if there are two mounted at the same time, which reveals the bug. If you change this so that foo counts its mounts/unmounts instead you'll see it mounts twice but will only unmount once even if you completely stop rendering it.
• Note the promise resolving on line 22 and then the selected state changing on line 23 which triggers the reactive statement on 32 and immediately creates a new promise. Changing anything about this causes the bug to disappear.

Logs

No response

System Info

System:
    OS: Linux 5.4 Linux Mint 20.3 (Una)
    CPU: (12) x64 AMD Ryzen 5 2600 Six-Core Processor
    Memory: 8.71 GB / 15.56 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 16.16.0 - ~/.nvm/versions/node/v16.16.0/bin/node
    npm: 8.11.0 - ~/.nvm/versions/node/v16.16.0/bin/npm
  Browsers:
    Chrome: 104.0.5112.79
    Firefox: 103.0.1

svelte@3.49.0

Severity

annoyance

Possibly related

#1591, #5322, #5944, #7806.

[Conduitry]

This should be fixed now in 3.51.0 - https://svelte.dev/repl/d3acebb52bf34f73a17d8afa2ddd19ce?version=3.51.0