vercel/next.js · error

Both ${item.domain} and ${defaultLocaleDuplicate.domain} con

Error message

Both ${item.domain} and ${defaultLocaleDuplicate.domain} configured the defaultLocale ${item.defaultLocale} but only one can. Change one item's default locale to continue

What it means

Error "Both ${item.domain} and ${defaultLocaleDuplicate.domain} configured the defaultLocale ${item.defaultLocale} but only one can. Change one item's default locale to continue" thrown in vercel/next.js.

Source

Thrown at packages/next/src/server/config.ts:1300

        if (!item || typeof item !== 'object') return true
        if (!item.defaultLocale) return true
        if (!item.domain || typeof item.domain !== 'string') return true

        if (item.domain.includes(':')) {
          console.warn(
            `i18n domain: "${item.domain}" is invalid it should be a valid domain without protocol (https://) or port (:3000) e.g. example.vercel.sh`
          )
          return true
        }

        const defaultLocaleDuplicate = i18n.domains?.find(
          (altItem) =>
            altItem.defaultLocale === item.defaultLocale &&
            altItem.domain !== item.domain
        )

        if (!silent && defaultLocaleDuplicate) {
          console.warn(
            `Both ${item.domain} and ${defaultLocaleDuplicate.domain} configured the defaultLocale ${item.defaultLocale} but only one can. Change one item's default locale to continue`
          )
          return true
        }

        let hasInvalidLocale = false

        if (Array.isArray(item.locales)) {
          for (const locale of item.locales) {
            if (typeof locale !== 'string') hasInvalidLocale = true

            for (const domainItem of i18n.domains || []) {
              if (domainItem === item) continue
              if (domainItem.locales && domainItem.locales.includes(locale)) {
                console.warn(
                  `Both ${item.domain} and ${domainItem.domain} configured the locale (${locale}) but only one can. Remove it from one i18n.domains config to continue`
                )
                hasInvalidLocale = true

View on GitHub (pinned to 258b1c1bc0)

Solutions

1. Change one domain's defaultLocale so each defaultLocale is used by only one i18n domain entry.

When it happens

Trigger: Thrown at packages/next/src/server/config.ts:1253 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of vercel/next.js@258b1c1bc0 (2026-08-19). Data as JSON: /api/errors/ac78afbdef20ca5a. Report an issue: GitHub.