When creating an email campaign, on the first Details step, you will see the option to create or select a "Send group".
You can also create and manage send groups from Campaigns > Send groups in the menu.
When you send a campaign that is part of a send group, only the subscribers who have NOT been sent any of the other campaigns in that same send group, will receive the campaign.
You can use a send group to send duplicate copies of the same campaign to different lists without sending again to any subscribers that might appear on multiple lists.
There is no time limit on a send group, you can use a send group days or even years after it was first created. So another way you can use send groups is to re-send a copy of a campaign to subscribers who may have been added to your list after that earlier campaign was sent.
