{"record":{"id":"06d311104e6c6f1f","repo":"vercel/next.js","slug":"specified-i18n-locales-contains-the-following-dupl","errorCode":null,"errorMessage":"Specified i18n.locales contains the following duplicate locales:\\n${[...duplicateLocales].join(', ')}\\nEach locale should be listed only once.\\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config","messagePattern":"Specified i18n\\.locales contains the following duplicate locales:\\\\n(.+?)\\\\nEach locale should be listed only once\\.\\\\nSee more info here: https://nextjs\\.org/docs/messages/invalid-i18n-config","errorType":"validation","errorClass":null,"httpStatus":null,"severity":"error","filePath":"packages/next/src/server/config.ts","lineNumber":1401,"sourceCode":"    if (!i18n.locales.includes(i18n.defaultLocale)) {\n      throw new Error(\n        `Specified i18n.defaultLocale should be included in i18n.locales.\\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`\n      )\n    }\n\n    const normalizedLocales = new Set()\n    const duplicateLocales = new Set()\n\n    i18n.locales.forEach((locale) => {\n      const localeLower = locale.toLowerCase()\n      if (normalizedLocales.has(localeLower)) {\n        duplicateLocales.add(locale)\n      }\n      normalizedLocales.add(localeLower)\n    })\n\n    if (duplicateLocales.size > 0) {\n      throw new Error(\n        `Specified i18n.locales contains the following duplicate locales:\\n` +\n          `${[...duplicateLocales].join(', ')}\\n` +\n          `Each locale should be listed only once.\\n` +\n          `See more info here: https://nextjs.org/docs/messages/invalid-i18n-config`\n      )\n    }\n\n    // make sure default Locale is at the front\n    i18n.locales = [\n      i18n.defaultLocale,\n      ...i18n.locales.filter((locale) => locale !== i18n.defaultLocale),\n    ]\n\n    const localeDetectionType = typeof i18n.localeDetection\n\n    if (\n      localeDetectionType !== 'boolean' &&\n      localeDetectionType !== 'undefined'","sourceCodeStart":1383,"sourceCodeEnd":1419,"githubUrl":"https://github.com/vercel/next.js/blob/34433fd12ee8074ea3f47af9f36255c7390d0301/packages/next/src/server/config.ts#L1383-L1419","documentation":"Error \"Specified i18n.locales contains the following duplicate locales:\\n${[...duplicateLocales].join(', ')}\\nEach locale should be listed only once.\\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config\" thrown in vercel/next.js.","triggerScenarios":"i18n.locales contains duplicate locale entries.","commonSituations":"The same locale listed twice in next.config i18n.locales.","solutions":["Remove the duplicate locale entries from i18n.locales."],"exampleFix":null,"handlingStrategy":"validation","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":[],"tags":[],"backgroundTag":null,"analyzedSha":"34433fd12ee8074ea3f47af9f36255c7390d0301","analyzedAt":"2026-08-19T02:10:13.922Z","contentChangedAt":"2026-08-19T02:10:13.922Z","schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}