If the same contact is added to two separate lists, they will receive both campaigns. This is not ideal if your lists are receiving the same campaign so you will want to remove them from one of the lists.
On the other hand, if you try to upload the same email address in the same list you will receive an error message to tell you that the contact wasn’t added they are already in the database.
You can learn more about how list segmentation works in Campaign Monitor here.
Last updated November 2021