The Sitecore to Salesforce integration, S4S, includes a mapping wizard to support Sitecore 9 Experience forms. The new wizard is more advanced (see LinkedIn article) than what is currently available in the S4S WFFM mapping wizard. One nice feature is called the Salesforce Field Matching Rule that prevents duplicates occurring in Salesforce.
When pushing data to Salesforce (from a Sitecore Experience Form) we usually expect to insert a record, or update an existing one if a matching record exists. Before this can happen, we need to define what matching means, for example, in the case of creating Salesforce leads. You usually update a lead record if the email address matches but consider a family or group who share a single email address! If you need to capture them as individuals in Salesforce, the matching criteria would need to be an email address and first name.
Because businesses have complex matching requirements the S4S mapping wizard for Sitecore Experience Forms can allow multiple fields to be selected to ensure duplicates are not created in Salesforce.
In most cases, this feature makes it unnecessary to create a custom save action to eliminate de-duplication.