If a person registers under two different email addresses, or purchases a product using a different email address, or checks out under the wrong email address, the system creates a new account for them. To merge these email addresses under one contact, you can add a field called “2nd Email” in People > Custom Fields > New Custom Field by choosing the Type as Email. Then add the second email to that field. The account will remain under the first email address, but the system will pull any transactions under the second email address into the same record.