Duplication of {#each} items with experimental async

[Coretteket]

Describe the bug

Since v5.43.0, some items in an {#each} block are duplicating in a specific scenario related to the experimental async syntax. All awaited items except the first appear a second time (eg. item 1, item 2, item 3, item 2, item 3). The items are not duplicated in the SSR ouput (in a SvelteKit app), the duplicated items are only added on client load, it appears.

Note: This only occurs in a production build, not in development. No errors/warnings are logged. The issue does not occur in v5.42.3 or below.

Reproduction

The following is a minimal reproduction of an issue I am encountering in a real app. The issue does not occur if const another = {...} (L5) is moved before const messages = await ... (L4), if another is not passed to the component, or if the <p>{message}</p> is defined directly in the {#each} block, instead of in a seperate component.

https://stackblitz.com/edit/sveltejs-kit-template-default-es5urw13?file=src%2Froutes%2F%2Bpage.svelte

Logs

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    svelte: 5.43.0 => 5.43.0

Severity

blocking an upgrade