Data Import Wizard matching options
Key Takeaway
In the Data Import Wizard, a row that matches an existing record by your matching type updates that record, and a row that doesn't match creates a new one. The matching type you pick is how you prevent duplicates.
When you import records with the Data Import Wizard, you pick a matching type. The wizard uses it to decide whether each row in your file is an existing Salesforce record or a new one. The choices differ by object and by how strictly they compare values.
How a matching type prevents duplicates
A matching type sets the criteria for avoiding duplicates.
For example, imagine you match leads by email. If a row in your file has the same email as an existing lead, the wizard updates that lead. If you don't match by email, the same row creates a new lead, even though that email already exists in Salesforce.