Duplicate identifier 'PagesPageConfig' when Building a Page Router NextJS Application without an pages/api route

Link to the code that reproduces this issue

https://github.com/b-jv-de/next-test-pagerouter/

To Reproduce

1. create a new nextjs application

npx create-next-app@latest next-test-pagerouter
Need to install the following packages:
create-next-app@16.2.9
Ok to proceed? (y) y

√ Would you like to use the recommended Next.js defaults? » No, customize settings
√ Would you like to use TypeScript? ... Yes
√ Which linter would you like to use? » ESLint
√ Would you like to use React Compiler? ... No
√ Would you like to use Tailwind CSS? ...  Yes
√ Would you like your code inside a `src/` directory? ...  Yes
√ Would you like to use App Router? (recommended) ... No 
√ Would you like to customize the import alias (`@/*` by default)? ... No
√ Would you like to include AGENTS.md to guide coding agents to write up-to-date Next.js code? ... No 
Creating a new Next.js app in C:\Projekte\bgrci\next-test.

• start development server npm run dev

• stop development server

• start building npm run build
  => no error

• delete the api folder next-test-pagerouter\src\pages\api (or rename it to e.g. api2)

• start development server npm run dev

• stop development server

• start building npm run build
  => error

npm run build

> next-test-pagerouter@0.1.0 build
> next build

✓ Running next.config.ts took 64ms
▲ Next.js 16.3.0-canary.76 (Turbopack)

  Running TypeScript  ...Failed to type check.

.next/dev/types/validator.ts:7:6
Type error: Duplicate identifier 'PagesPageConfig'.

   5 |
   6 |
>  7 | type PagesPageConfig = {
     |      ^
   8 |   default: React.ComponentType<any> | ((props: any) => React.ReactNode | Promise<React.ReactNode> | never | void)
   9 |   getStaticProps?: (context: any) => Promise<any> | any
  10 |   getStaticPaths?: (context: any) => Promise<any> | any
Next.js build worker exited with code: 1 and signal: null

• delete the .next folder
• start building npm run build
  => no error

Current vs. Expected behavior

I expected no error on typescript validation.
The error occurs when there is no api route on build when the development mode was started before.

When the api route exists you can run dev/build without an issue.
When you delete the .next folder before building the error also doesn't happen.

Provide environment information

perating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 15151
  Available CPU cores: 12
Binaries:
  Node: 24.5.0
  npm: 11.5.1
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.3.0-canary.81 // Latest available version is detected (16.3.0-canary.81).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

TypeScript

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

next build (local)

Additional context

No response

Start with the linked next-test-pagerouter reproduction and follow the dev-then-build sequence after removing src/pages/api. Inspect the generated .next/dev/types/validator.ts where PagesPageConfig is duplicated. Done means next build passes without deleting .next, both with and without the pages/api route.