Remove duplicates for subscribers on multiple lists in Marketing Cloud Email

Marketing Cloud removes duplicate subscribers based on the order you selected the lists. The order in which you select the lists can be important for tracking and analytics. 

For example:

If you select List A, List B, and List C for a send and the unique subscriber is on all three lists, Marketing Cloud sends to the subscriber as part of List A. The send ignores them on List B and List C. The same is true for the tracking related to that send. If you review tracking by lists, it will count the subscriber's tracking data as part of the List A, but not List B or List C.

When using the Guided Send Wizard, the Selected Box at the bottom of the window displays the lists in the order you selected them. You can use this feature to ensure the lists are being used in the correct order.

Lists, groups, and data filters created from a list are always de-duplicated at the time of send by SubscriberID. 
 

Important note:

When your account uses the Subscriber Key as the unique identifier, the same email address exists in your subscriber list for many unique Subscriber Keys. In this instance, the send removes duplicates on the Subscriber Key field which is your duplicated, unique value.