Within the platform there is the ability to store duplicate email addresses across multiple Marketing Lists. Don't know what a Marketing List is? Check here.
There are many use-cases in which the end-user may wish to keep firstname.lastname@example.org within multiple Marketing lists, each used for a distinctive purpose. The most common use-case would be brand management. It's a very real possibility that email@example.com may be interactive with your organization across multiple brands, and as such you may wish to market to firstname.lastname@example.org differently by brand. Thus, we have the option to allow duplicate email addresses across multiple Marketing lists.
To begin, navigate to Manage Settings--->Advanced Settings--->Data Management.
From this screen the user will be shown 3 options. This article highlights option 1, and so has trimmed the settings screen down to reflect this option only.
By default, this setting will be on 'Remove Duplicates'. If you wish email@example.com to be shown as well as the ability to be uploaded into multiple lists, select 'Do Nothing'. Moving forward, when a new list is uploaded or an existing list is updated, firstname.lastname@example.org will now appear across any Marketing lists they are a part of.