Link to the code that reproduces this issue

https://github.com/dkokotov/nextjs-edge-request-cloning-bug

To Reproduce

1. Start the application in development
2. Navigate to http://localhost:3000
3. Observe that the method and headers of request 2 do not match those of request 1, even though request 2 was created by cloning request 1 (const req2 = new Request(req1))

Current vs. Expected behavior

Request 2 method and body should be the same as those of request 1.

Note this only happens for edge runtime. If, in the example below copied from the repro repository), I comment out the export const runtime = 'edge', the output is as expected.

export default function Home() {
  const req1 = new Request('https://example.com', {
    method: 'POST',
    headers: { 'X-Example-Header': 'Foo' }
  });
  const req2 = new Request(req1);

  return <div>
    Request 1 method: {req1.method}
    <br/>
    Request 1 headers: {req1.headers}
    <br/>
    Request 2 method: {req2.method}
    <br/>
    Request 2 headers: {req2.headers}
  </div>
}

export const runtime = 'edge';

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:43 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T6000
Binaries:
  Node: 20.8.0
  npm: 10.1.0
  Yarn: 1.22.19
  pnpm: 8.7.5
Relevant Packages:
  next: 14.0.2-canary.5
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: N/A

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

App Router, Middleware / Edge (API routes, runtime)

Additional context

Seems to only happen for running in dev mode. If I deploy the repro example to Vercel (https://nextjs-edge-request-cloning-bug.vercel.app), the output is as expected.