'The domain has been registered already' when adding a domain to Resend

Not specified by the original author.

Symptom: adding a domain in the dashboard fails with a message like "The example.com domain has been registered already".

1. Confirm the cause: this means the domain is active on a different Resend account. Common reasons: a teammate or contractor added it, or you are logged into a different account than the one that first added it.
2. First check: ask your team whether someone already uses Resend, and double-check which account you are signed into. The cheapest fix is using the account that already has it.
3. Fix via claim: on the Domains page, add the domain again and click Start claim when the in-use prompt appears. Resend gives you a DNS TXT record to add.
4. Add that TXT record at your DNS provider to prove ownership. The claim expires in 7 days, so do not let it sit.
5. If you previously started a claim, re-submit the same domain and click Continue claim to pick up where you left off.
6. Verify: once the claim completes, the domain appears in your account and you can add the normal sending records. If you cannot add DNS records, contact Resend support.

Documented in Resend knowledge base: a domain can only be active on one Resend account; the self-serve claim flow uses a DNS TXT proof record and expires in 7 days. Evidence: resend.com/docs/knowledge-base/domain-already-registered.