Requiring a unique email address is something that is set in the "membership" section of the web.config file.
<membership>
<providers>
<clear />
<add connectionStringName="SiteSqlServer" enablePasswordRetrieval="true" enablePasswordReset="true" requiresQuestionAndAnswer="false" minRequiredPasswordLength="7" minRequiredNonalphanumericCharacters="0" requiresUniqueEmail="true" passwordFormat="Encrypted" applicationName="DotNetNuke" description="Stores and retrieves membership data from the local Microsoft SQL Server database" name="AspNetSqlMembershipProvider" type="System.Web.Security.SqlMembershipProvider" />
</providers>
</membership>
If you add this requirement to an existing site, you should removed or change any duplicate emails prior to changing web.config.