public class Student
{
    public string StudentId { get; set; }
    public string Gender { get; set; }
    public List<EmailAddresses> emailAddresses { get; set; }
}

public class EmailAddresses
{
    public string EmailAddress { get; set; }
}

StudentId   EmailAddresses  IsPreferred Number  TelephoneType
123456789   maryjoe@gmail.com   FALSE   5556565 Present Evening Phone
123456789   maryjoe@gmail.com   FALSE   8885566 Permanent Day Phone
123456789   mary.joe@cuu.edu    TRUE    5556565 Present Evening Phone
123456789   mary.joe@cuu.edu    TRUE    8885566 Permanent Day Phone
456789123   dh@mycollege.edu    TRUE    7513150 Business Day Phone
456789123   donna.hill@cu.edu   TRUE    4123300 Present Day Phone
456789123   donna.hill@cu.edu   FALSE   4123300 Present Day Phone

public List<Student> GetStudentData()
{
    List<Student> dataStudent;
    using (IDbConnection connection = RepositoryHelper.OpenConnection())
    {
        dataStudent = connection.Query<dynamic>(
            "mystoredprocedure",
            commandType: CommandType.StoredProcedure)
                .GroupBy(x => x.StudentId)
                .Select(x => new Student
                    {
                        StudentId = x.First().StudentId,
                        Gender = x.First().Gender,
                        emailAddresses = x.Select(ea => new EmailAddresses
                            {
                                EmailAddress = ea.emailAddresses
                            }).ToList()
                    }).ToList();

        return dataStudent;
    }
}