There is no single button that duplicates an entire campaign – recipients, subject line, schedule and tracking settings always have to be entered again. The email design, though, can be duplicated directly:

1. Clone a template outright – go to Marketing > Email > Templates, find the template (its name/ID is shown on the campaign page under Content), click the clone/copy icon in its row and confirm. This creates an exact new template with the same design (and the same title) that you can attach to any campaign.
2. Reuse a design while building a new campaign – in an existing campaign's Content step, open the template editor and turn on Save as Template in its top bar. That design then appears under "Start from scratch or choose a template" the next time you create a campaign; picking it there automatically creates a fresh copy of it for the new campaign.