It does this by keeping track of which constituent records have received an email. So if you have duplicate constituent records with the same email address, that email address can receive the same email twice. When you take an existing constituent record and "Create As Admin" to create an admin record, it does basically create a set of duplicate record. This does mean that the email address associated can receive the same email twice. If this is a problem, edit the admin record and mark it to not accept email to resolve the problem.