In the first step of the Campaign Builder, an Exclude box exists to avoid sending one particular customer multiple offers at the same time (or with overlapping durations). This consequently allows for a clean measurement of your campaign performance.
As displayed, this checked box will exclude customers included in other concurrent actions.
The Campaign Duration defined for each campaign is used to determine when campaigns overlap for this purpose. That is, when scheduling a campaign for a specific target group, a customer may already appear in a different target group in a previously scheduled campaign, with duration still in effect. The Exclude criterion removes customers included in other current campaigns from receiving the new one.
Learn more here.