Business etiquette


I recently was emailed by a prospective client for a work opportunity. Another client who had recommended me was cc’d on the same email.
I promptly “replied all” and sent over a proposal letter with the fee and my interest in the job. I then received an email back the next day from my existing client. The one who recommended me. However in that email she started the email by addressing someone else as if perhaps she sent it to me by accident. The email said that she had highly recommended me (or other person I guess) and looked forward to working together again soon.
So how do I respond to this email without being confrontational yet also finding out who the other person is and if the clients are going with them instead?
I’d be grateful to get some guidance here. Thank you!