You cannot merge via smart campaign because you will not be able to view the conflicted fields where you need to choose one value to overwrite. Merging leads can only be done one at a time by selecting two leads from a list view and running the "Merge Leads" lead action, or through your CRM, which is preferred.
In addition to what Devraj said, this process is best done with a service like RingLead or Demand Tools.
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Adding Openprise to the list of merging platforms