Specified i18n.locales contains the following duplicate loca

Error message

Specified i18n.locales contains the following duplicate locales:
${[...duplicateLocales].join(', ')}
Each locale should be listed only once.
See more info here: https://nextjs.org/docs/messages/invalid-i18n-config

What it means

Thrown at config.ts:1330 when `i18n.locales` contains case-insensitive duplicates. The code lowercases each locale and adds it to a `normalizedLocales` set; if a lowercased form is already present, the original-casing value is recorded in `duplicateLocales`. The error lists the duplicates and requires each locale to appear once.

Source

Thrown at packages/next/src/server/config.ts:1331

    if (!i18n.locales.includes(i18n.defaultLocale)) {
      throw new Error(
        `Specified i18n.defaultLocale should be included in i18n.locales.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    const normalizedLocales = new Set()
    const duplicateLocales = new Set()

    i18n.locales.forEach((locale) => {
      const localeLower = locale.toLowerCase()
      if (normalizedLocales.has(localeLower)) {
        duplicateLocales.add(locale)
      }
      normalizedLocales.add(localeLower)
    })

    if (duplicateLocales.size > 0) {
      throw new Error(
        `Specified i18n.locales contains the following duplicate locales:\n` +
          `${[...duplicateLocales].join(', ')}\n` +
          `Each locale should be listed only once.\n` +
          `See more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    // make sure default Locale is at the front
    i18n.locales = [
      i18n.defaultLocale,
      ...i18n.locales.filter((locale) => locale !== i18n.defaultLocale),
    ]

    const localeDetectionType = typeof i18n.localeDetection

    if (
      localeDetectionType !== 'boolean' &&
      localeDetectionType !== 'undefined'

View on GitHub (pinned to 0ae8c72462)

Solutions

Remove the duplicate locale entries so each appears exactly once.
If duplicates arise from case differences, standardize on a single canonical casing.
Dedupe programmatically when building the list: `locales: [...new Set(allLocales.map((l) => l))]` (and normalize case first if needed).

Example fix

// before
i18n: { locales: ['en-US', 'EN-US', 'fr'], defaultLocale: 'en-US' }
// after
i18n: { locales: ['en-US', 'fr'], defaultLocale: 'en-US' }

Defensive patterns

Strategy: validation

Validate before calling

const lower = i18n.locales.map((l: string) => l.toLowerCase());
if (new Set(lower).size !== lower.length) throw new Error('duplicate (case-insensitive) locales');

Type guard

function noCaseInsensitiveDuplicates(locales: string[]): boolean {
  const lower = locales.map((l) => l.toLowerCase());
  return new Set(lower).size === lower.length;
}

Prevention

Normalize locale casing upstream (e.g. always canonical case).
Dedupe with `Array.from(new Set(locales.map((l) => l)))` before exporting.

When it happens

Trigger: `locales: ['en-US', 'EN-US']`; `locales: ['fr', 'FR', 'Fr']`; `locales: ['pt-BR', 'pt-br']`. Mixed casing is treated as the same locale.

Common situations: Merging locale lists from multiple teams/modules without deduplication; CMS-driven locale config that injects variants; copy-paste duplication.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/67728b524413e695. Report an issue: GitHub.