Can all contacts that share an email be unsubscribed at once?
Short answer: no. 

An unsubscribe request is handled here:

https://github.com/civicrm/civicrm-core/blob/master/CRM/Mailing/Form/Unsubscribe.php

The parameters to an unsubscribe request are translated into the corresponding contact, not the email address.

So what you're asking would require the code to scan the database for contacts with the same email address to unsubscribe, which in some circumstances wouldn't be the desired result.

Conceptually, unsubscribes/subscribes are stored/handled by contact id, not email address.

Your options would be to write your own unsubscribe handler, or (perhaps) to use civirules.