Enable unique email for the entire participant list To enable unique email across your entire event, and thus prevent two participants from different populations from registering with the same email address. Go to the Event > Advanced options tab and check the first box under Unique email before saving your changes.
If you want to enable unique email for one or more specific populations, uncheck the box mentioned above and check the populations for which you want to prevent registrations with the same email address. In the example below, I prevent all my populations from registering twice with the same email address; however, I allow orders to be placed multiple times with the same email address.
Finally, you can enable this option at the level of one or more categories of your choice. To do so, go to the registration flow of a category and click the Edit this registration category button. Check the Prevent two registrations with the same email address box and save your changes.