If you have customers who are signing up multiple times and creating duplicate submissions, you can enable a feature to prevent this with the Prevent Duplicate Signups setting.
Go to Settings > Site Settings > Site Details and click the check box next to Prevent Duplicate Signups
How it works
When this setting is on, we will look to see if any people on your list are using the same email. If the person with the same email address is not part of the current family, and they have a family member who is actively on the list, they will see the below error.