I use this for checking an existing emailId in my table and inserting it...It works fine how to show message to user when he tries to register with an existing mailId....

if (!taxidb.Registrations.Where(u => u.EmailId == reg.EmailId).Any())
{
     taxidb.Registrations.InsertOnSubmit(reg);
     taxidb.SubmitChanges();
}

and my controller has this,

 RegistrationBO reg = new RegistrationBO();
 reg.UserName = collection["UserName"];
 reg.OrgName = collection["OrgName"];
 reg.Address = collection["Address"];
 reg.EmailId = collection["EmailId"];
 reg.Password = collection["Password"];
 reg.CreatedDate = System.DateTime.Now;
 reg.IsDeleted = Convert.ToByte(0);
 regrep.registerUser(reg);

Any sugesstion how to show "EmailID" already exists to the user with asp.net mvc...