When you send an email to multiple lists or segments, our system checks for duplicate email addresses so that no one receives the same campaign twice.
Each list an active subscriber is on counts towards your subscriber total for monthly billing, whether they are sent to or not.
Dynamic content
The deduplication described above can affect emails that include dynamic content. For example, say you have two sections, one that is visible to "List A", and another that is visible to "List B". If a recipient is on both of those lists, they will only receive either the "List A" or the "List B" version, they will not see the content relevant to both. It is not possible to choose which version a subscriber on multiple lists should receive.
Segments behave differently. For example, say you have two sections in an email, one that is visible to "Segment A", and another that is visible to "Segment B". If a recipient is a member of both of those segments, they will only receive one email, however they will see both "Segment A" and "Segment B" content.
Campaign reports
In a multiple list send for a campaign, where a subscriber belongs to more than one list, subscriber activity for the sent campaign will only be recorded for one of the lists. It is not possible to choose which list subscriber activity is recorded against.
In a multiple segment send, where a subscriber belongs to more than one of the segments sent to, the "Lists & segments" report will show activity in all of those segments.