Next internationalized routing allows multiple locales in url path

What version of Next.js are you using?

11.0.1 and 12.0.0

What version of Node.js are you using?

14.18.1

What browser are you using?

Firefox, Chrome

What operating system are you using?

macOS

How are you deploying your application?

next dev

Describe the Bug

When adding internationalized routing to a Next.js app, multiple subpaths redirect to the same route.
If for example I use the "en" locale as the defaultLocale, the following urls route to the same index page:

• http://localhost:3000
• http://localhost:3000/en
• http://localhost:3000/en/en
• http://localhost:3000/[anyOtherLocaleInNextConfig]/en

The router object for the http://localhost:3000/en/en option looks similar to this:
{ "pathname": "/", "route": "/", "query": {}, "asPath": "/en", "components": { "/": { "initial": true, "props": { "pageProps": {} } }, "/_app": { "styleSheets": [] } }, "isFallback": false, "basePath": "", "locale": "en", "locales": [ "de", "en", "us" ], "defaultLocale": "en", "isReady": true, "isPreview": false, "isLocaleDomain": false, "events": {} }

This causes problems when using slugs like http://localhost:3000/locale/[:slug] because the second /en in the http://localhost:3000/en/en url doesn't get detected as a slug. However using any string other than a locale defined in the next.config.js works just fine.

I hope this description is comprehensible - I've been banging my head against my desk for the past few days and might have lost one or two braincells.

Expected Behavior

When using slugs like http://localhost:3000/locale/[:slug], the second /en in localhost:3000/en/en should be detected as the slug.

To Reproduce

1. Setup a test repository using yarn create next-app --typescript
2. Add the following i18n config to the next.config.js: i18n: { defaultLocale: "en", locales: ["en", "de", "us"] },
3. Start the development server using yarn dev
4. Visit http://localhost:3000/en/en, http://localhost:3000/en, http://localhost:3000 and see the exact same page
5. Add a [slug] to the pages folder and create an index file in the [slug] folder
6. You can access the page using http://localhost:3000/en/test but not using http://localhost:3000/en/en, http://localhost:3000/en/de or http://localhost:3000/en/us
7. You can also acces the page using e.g. http://localhost:3000/en/en/test which then redirects to just http://localhost:3000/en/test

Reproduce the behavior with the i18n settings in next.config.js, then add the [slug] page under pages and run the app with yarn dev. Verify that /en/en treats the second en as the slug without redirecting, while the other documented locale routes continue to behave as expected.