By enabling this option you can prevent participants from submitting more than one entry for your project by using the same email address. Once this option is active, it will check if an email address that was already entered was submitted again for the same project.
You can find this option in the Project Configuration page, under the "Personal Data" section.
You can turn this option on or off anytime to change this behavior. The alert message your participant will see is predefined by the language setting for the project.