sendmail aliases – send to root plus external address?

There are many examples of how to use aliases to send emails to multiple addresses. However, I am interested in sending to an internal name and an external address.

This does not work, for example, but the external address receives base messages:

root: root, SomeOtherAddress @ email.com

I also tried this, but I'm denied access trying to attach to the file:

root: /var/mail/root,SomeOtherAddress@email.com

I want to record root messages where they would normally be and also send a copy to an external address.

Proposals?

Many Thanks.