A: With Email campaigns, although a contact may be added several times (they may be added to several groups which are added in) they should only be sent the campaign once. If the campaign is re-created (e.g. by using past emails sent) then they can added in to that as that is a fresh campaign.
Likewise, with automations a contact with their unique email address will only be able to pass through the automation once. If you duplicate the automation then that becomes a new automation and the same contacts added to the original will be able to be added to the duplicate automation.