If you are the CA, you can sign a new certificate yourself for google.com and the browser will accept it. It’s effectively allows MITM for any certificate. Worse, it’s not even limited to certificates under that CA. The browser has no way of knowing there’s 2 “valid” certs at once, and in fact that is allowed regardless (multiple servers with different instances of the SSL cert is a possibility).

Certificate pinning might save things, since that will force the same certificate as was previously used, but I’m not sure this is a common default.