Quite a few people mistype their email address when entering their information through the website. They might wait and wait for an email confirmation, resend it, and then try again by entering the right email. This is a great way to reach more than one person. If a client makes a mistake when opting in do not delete the email address. Often it is not an invalid account and offers a way to reach more people.
It can be tempting to fix the address; however, you actually have a chance of picking up someone new. Email addresses are private and it is a delicate situation not to be considered spam. The key is to include an opt-out and a "sorry if this reaches you in error" message. If the email is invalid you can delete it. Always delete any subscriber asking to be removed, so that you keep your good business name.