The best way to avoid creating duplicate records when importing a file that may contain both new and existing data is to use an "upsert" operation. The Data Import Wizard provides a user-friendly interface for this. It allows the sustainability manager to map columns from the file to Salesforce fields and specify a matching field (like an external ID or a unique bill identifier). The wizard then checks if a record with that identifier already exists. If it does, the existing record is updated; if not, a new record is inserted.

A. The Merge Leads function is a manual tool used to combine duplicate Lead records after they have already been created, not for preventing duplicates during import.

B. Validation rules prevent records from being saved if they don't meet criteria. While they can be used to block duplicates, they are less efficient for bulk imports than an upsert operation.

C. Using "Insert" with Data Loader will always create new records, which is the direct cause of duplicates if the records already exist in the system.

1. Salesforce Help. (2023). Import Data with the Data Import Wizard. "The Data Import Wizard provides a unified interface that lets you import data for... custom objects. It supports... updating existing records, and inserting new records." This describes the upsert functionality.

2. Salesforce Help. (2023). Decide Which Data Import Tool to Use. The Data Import Wizard is recommended for users importing fewer than 50,000 records and who need to prevent duplicates by matching on fields like name, email, or ID.