This feature is very useful when you do not want to have duplicate or multiple registrations from the same person. In a free event, it is common that the same person registers twice or more because they may forget that they have registered. It will be quite troublesome for the organizers to have duplicate records in the database.
In order to avoid that, you can simply switch on this feature and set which registration data will be set as a unique field by the system and the same data cannot be used to register again.
To enable this feature,
Go to My Events>> Edit Event>> Setup>> Other Settings>> Advanced Settings.
You will see the Unique Registration Rule and click on the Edit button.
On the next page, check the field that you want to be unique. (You can check more than one filed).
So, when the same person tries to register using the same email in this case, an error message will appear like this.