I understand that you are trying to verify custom domain for your organization, but you have received an error message stating that the domain has already been registered for another tenant.
If the domain is registered in other tenant, and if you trying to verify the same domain in different tenant it will not verify, because Azure does not allow to use same domain in multiple tenants so, if you want to use the domain in your tenant it needs to remove the domain from the other tenant.
If you are trying to find the details of custom domain, you can find domain registrar details by following the bellow document.
Find your domain registrar If you have received no response, you need create a ticket with Microsoft support team with the tenant ID by inform them stating as that you are an owner of the custom domain and unable to verify custom domain for organization because the domain is verified in another tenant.
Once the ticket is created with support team you will have to work with our data protection team and validate your identity against your tenant for security purpose post that this team will help you on your issue.
Hope this helps. Do let us know if you any further queries.
Reference: https://learn.microsoft.com/en-us/entra/identity/users/domains-admin-takeover
Thanks,
Akhilesh.
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