Explanation
Two options that would prevent the customer from duplicatingrecords when implementing a Sales Cloud instance are:
Get existing records updated with new Keys sourced from Sales Cloud instance. This option will allow the customer to update their existing records in Marketing Cloud with new keys that match their SalesCloud records, such as Contact ID or Lead ID. This will ensure data consistency and avoid duplication across both systems.
Continue as normal, as Marketing Cloud Contact Models will dedupe keys by Email Address. This option will allow the customer to continue using Email Address as their primary key in Marketing Cloud, and rely on Marketing Cloud Contact Models to dedupe keys by Email Address when syncing data from Sales Cloud. This will ensure data accuracy and avoid duplication within Marketing Cloud.
Uploading CSV with Migrated Subscriber Keys to All Subscribers will not prevent duplication, as it will create new records in Marketing Cloud with different keys than the existing records. Purging the current records and carrying on with new keys sourced from Sales Cloud will result in data loss and potential compliance issues. References:
https://help.salesforce.com/s/articleView?id=sf.mc_co_subscriber_key.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_co_contact_key.htm&type=5