The unique email constraint determines whether constituents can register with an email address that is already in the system. However, because this can sometimes be a problem for calendar events, personal fundraising, and TeamRaiser there are separate settings that will override the site wide setting for each module respectively. By default, duplicate email addresses are allowed for calendar registrations, TeamRaisers, and Personal Fundraising but this setting can be changed. Contact support to have the override turned off. Please note that this can cause problems in the situation where family members register each other for an event who don't have their own email address.