When an Applicant sends in another Application, there is usually no reason to create another Applicant. Like if the same applicant applies for two different positions. For that reason there is a Duplicate detection built into the solution that triggers based on the Email address of the applicant.
In some cases though, several applications from different applicants might be submitted with the same email address. This can for example be the case if an application is submitted via stepstone or the several applicants are represented by the same Recruiting Agency. Of course, those Applications should not be blocked by the Duplicate Detection. For that reason, you can exclude email addresses from being blocked by the duplicate detection. This tutorial shows you how to do that.