Salesforce upserts data based on the external ID values. A

masking

task creates fields for the external IDs in the target when you configure the task. If the external ID field is empty, Salesforce creates duplicate records. To avoid duplicate records in target for the upsert operation, you need the external IDs that the

masking

task creates. You can update the partial sandbox to load the external IDs to perform an upsert operation.

You can first update the partial sandbox to add the external IDs, and update the external ID values for the existing records in the target. Run the task with an upsert operation to update or insert records.