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://local…