vercel/next.js · error

Both ${item.domain} and ${domainItem.domain} configured the

Error message

Both ${item.domain} and ${domainItem.domain} configured the locale (${locale}) but only one can. Remove it from one i18n.domains config to continue

What it means

i18n domains validation in assignDefaultsAndValidate: while checking each i18n.domains entry, a locale in item.locales was also configured on another domainItem in the list. A locale may be assigned to only one domain; the warning names both domains and the duplicated locale and asks for its removal from one of them so routing is unambiguous.

Source

Thrown at packages/next/src/server/config.ts:1315

        )

        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
                break
              }
            }
          }
        }

        return hasInvalidLocale
      })

      if (invalidDomainItems.length > 0) {
        throw new Error(
          `Invalid i18n.domains values:\n${invalidDomainItems
            .map((item: any) => JSON.stringify(item))
            .join(
              '\n'

View on GitHub (pinned to 258b1c1bc0)

Solutions

1. Remove the duplicated locale from one of the i18n.domains entries; a locale can only belong to one domain.

When it happens

Trigger: Thrown at packages/next/src/server/config.ts:1268 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/26dd733e7d5f2760. Report an issue: GitHub.