using System.Collections.Generic;

using System.Linq;

using System;

public class Program

{

    public static void Main()

    {

            var table = new List<User>

            {

                new User("Jan", "Kowalski"),

                new User("Karol", "Kowalski"),

                new User("Wojtek", "Kowalski"),

                new User("Wojtek", "Kowalski"),

                new User("Michal", "Kowalski"),

                new User("Jan", "Kowalski"),

            };

​

            var names = table

                .GroupBy(u => u.Name)

                .Where(g => g.Count() == 1)

                .Select(g => g.Key);

        foreach (var name in names)

            {

                Console.WriteLine(name);

            }

    }

    class User

    {

        public User(string name, string surname)

        {

            Name = name;

            Surname = surname;

        }

        public string Name { get; set; }

        public string  Surname { get; set; }

​

    }

}