If someone makes a donation using an email address that is already in the system (and no other duplicate information) their donation will show up on the original record and will not create a new record for the person, even if unique email addresses are not enforced.
Donations are treated differently than other places in Convio where email address matching is done. In Donations, when the donation is made, if a match can be made on the email address, the transaction is applied to that constituent, regardless of the settings for enforcing unique email addresses. Since email address is used as a unique key, even if all other constituent information is different the donation will be recorded on the oldest record with that email address. This behavior is different from all other ways that constituent records are created.