layout.tsx renders twice when using SSR and parallel routes

Link to the code that reproduces this issue

https://github.com/eiiot/next-routes-rerendering

To Reproduce

1. Build the application with next build
2. Run with next start

Current vs. Expected behavior

RootLayout rendered is logged twice per request when it should only be logged once. This causes a bunch of issues, including fetching data from APIs twice.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:03:11 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6020
  Available memory (MB): 32768
  Available CPU cores: 12
Binaries:
  Node: 20.11.0
  npm: 10.2.4
  Yarn: 4.1.0
  pnpm: N/A
Relevant Packages:
  next: 15.2.0-canary.71
  eslint-config-next: 15.2.0-canary.71
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Performance, Parallel & Intercepting Routes

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local), Vercel (Deployed)

Additional context

This seems to be a fairly old issue: see this reddit thread discussing v14.

Start with the linked reproduction repository and inspect layout.tsx. Run next build followed by next start, then observe the RootLayout rendered logs for a request using parallel routes. The issue is done when the layout renders once per request and the reproduction no longer shows duplicate API fetching.