Hello @stanl3 !
Our system has a way of recognizing the email address when it is added into the contacts so that if it is ever re-added we only update the information instead of creating a duplicate contact. The same applies to sending to multiple email lists.
If one contact is on multiple lists and all are selected when sending an email they will only receive the email once. Lists are de-duplicated upon send and each email address receives the campaign only once.