According to book.civicrm.org:
Remove duplicate emails: CiviCRM will always dedupe your mailing based on unique contact records. For example, if a contact is in three of the groups you are including in your mailing, they will only be sent one copy of the email. However, if the same email is used by multiple contacts, that email address will receive multiple copies of the email—one for each contact using that address. Checking this box will ensure only one email is sent to each address
We do have some different contacts that use the same email address.
We would like to set up CiviMail so that it always removes duplicate email addresses and take it out of folk's hands.
I see the check box on the create a new mailing page but would always like this behavior. In fact, I can't see why you wouldn't want it to always remove them.
Is anyone aware of a way to set this as a default?