Unfortunately, two accounts cannot have the same email address associated with them, regardless of whether they are a client account or a staff account. However, if your new staff member/old client uses a Gmail account, you can simply add +1 before the @ symbol within their gmail address to create the new staff account and they will receive emails as normal to their original gmail account. Eg. if the account requests a password reset, the original gmail address will receive the password reset email despite the fact the email address has the +1 included in it.  If they do not have a gmail account they will need to utilise a completely different email address to create the staff login.