Your data includes duplicate records across legacy systems.

To prevent the creation of duplicate records in Salesforce during data migration from legacy systems, several tools and features should be employed in concert:A. Matching rules help define the criteria for identifying duplicates.B. Data Import Wizard includes settings to prevent duplicates during the import process.C. Workflow Rules can be configured to automatically trigger deduplication logic when specific conditions are met.Using these tools together enhances the prevention of duplicates, ensuring better data quality and integrity post-migration.

References: https://help.salesforce.com/articleView?id=data_importer_preventing_duplicates.htm