A new Marketing Cloud (MC) customer wants to now implement a Sales Cloud instance to...

To prevent duplicate Contact records, the customer needs to update the current All Subscribers list and Contact model by migrating the Subscriber Key to match the Sales Cloud ContactID or LeadID (B).

Uploading a CSV with the new Subscriber Keys mapped appropriately will ensure that existing records are updated (A).

Simply continuing as normal (C) will not prevent duplicates because Email Address is not unique enough across Salesforce objects. Full purge (D) is unnecessary and risks data loss.

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Exact Extracts from Salesforce Documentation:

Source: Salesforce Help – Subscriber Key and Contact Key Best Practices

→ "When integrating Sales Cloud, update existing Marketing Cloud subscribers to match the Salesforce ContactID or LeadID as Subscriber Key to prevent duplicate contacts."

Source: Salesforce Implementation Guide – Marketing Cloud Connect Setup

→ "Migrating existing contacts requires careful key management to align records across systems without duplication."

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