We'll explain the difference between the two Journey actions Add contact to campaign and Update lead.

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When you use Add contact to campaign, the Journey will copy the lead from the original campaign and create a duplicate on the new campaign. This means that you're keeping the lead on the first campaign while simultaneously creating the exact same lead on a new campaign.

When you use Update lead, the Journey will move the lead from the original campaign to the new campaign (so no duplicates at play here) - if you, of course, choose to update the campaign. You can also choose to update the status, last called by, call attempts, etc. This action will always keep and update the original lead.
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NB: If you decide not to further alter any of the other fields - the information in those fields will remain the same.
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