Setting cache-control header for icon routes results in duplicate header values

[ppiwo]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/quirky-bassi-x3nw3t

To Reproduce

1. Open codesandbox link
2. Open next.config.mjs - note that the cache-control header is being overridden
3. Open preview in a new tab
4. Navigate to /favicon.ico
5. View response headers

Current vs. Expected behavior

Expected: cache-control: max-age=1234
Actual: cache-control: max-age=1234, public, max-age=0, must-revalidate

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.0.0-canary.154 // Latest available version is detected (15.0.0-canary.154).
  eslint-config-next: N/A
  react: 19.0.0-rc-94e652d5-20240912
  react-dom: 19.0.0-rc-94e652d5-20240912
  typescript: 5.3.3
Next.js Config:
  output: N/A

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

Middleware, Runtime

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

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

Additional context

Overriding the cache-control header in middleware.ts has the same result. This only seems to happen to icons.

[cbratschi]

Duplicate of #64864