To prevent the data import from creating duplicates when importing records from a system outside of Salesforce, you should use External ID. External ID is a custom field type that can be used to store unique identifiers from other systems. You can use External ID to match records during data import and avoid creating duplicate records in Salesforce. You can also use External ID to enable upsert operations, which can insert new records or update existing records based on the External ID value. Verified Reference: : Salesforce Help Article [5] : Salesforce Help Article [6]