Link to the code that reproduces this issue

https://github.com/ebanek/identifier-e-already-declared-nextjs-minification/

To Reproduce

To Reproduce

In a default npx create-next-app@latest my-app --yes.

repro.js

var join, run;
join = (e, t) => e + t,
  run = (rows) => {
    rows.map((s) => join(s.g, s.r));
    return rows.map((s) => (i) => join(s.g, i.l));
  };
export { run };

app/page.tsx

import * as M from "../repro.js";

export default function Home() {
  return <pre>{Object.keys(M).join(", ")}</pre>;
}

running "npm run build" gives:

 Collecting page data using 5 workers  .Error [ChunkLoadError]: Failed to load chunk server/chunks/ssr/[root-of-the-server]__0f7v31e._.js from runtime for chunk server/app/page.js
    at Object.<anonymous> (.next/server/app/page.js:2:3) {
  [cause]: SyntaxError: Identifier 'b' has already been declared
      at <unknown> (.next/server/chunks/ssr/[root-of-the-server]__0f7v31e._.js:1)
}

> Build error occurred
Error: Failed to collect page data for /
    at ignore-listed frames {
  type: 'Error'
}

This is a minimal example that produces the error I have.
It came from importing superdoc library, which is already minified. Everything works if I set --no-mangling for the next build in package.json.

Current vs. Expected behavior

I expect that the above example displays an empty page. Here I get a syntax error when running npm run build.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:56 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 14
Binaries:
  Node: 24.18.0
  npm: 11.16.0
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.3.1 // Latest available version is detected (16.3.1).
  eslint-config-next: N/A
  react: 19.2.8
  react-dom: 19.2.8
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Webpack, SWC, Turbopack

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

next build (local)

Additional context

No response