Are you getting this exception with this code?

MembershipUser user = Membership.CreateUser([username], [password]);

That exception message is a very poor description of the actual problem. You definitely have set requiresUniqueEmail = "true" in your web.config. The issue at hand is the method you have chosen. If you have requiresUniqueEmail set to true, then you must supply a unique email when you create a user. So you should use the appropriate overloaded method:

MembershipUser user = Membership.CreateUser([username], [password], [email]);

That should solve the problem, hopefully .