public static List<T> RemoveDuplicates<T>(List<T> items)
{
   return (from s in items  select s).Distinct().ToList();
}

public static void TestRemoveDuplicates()
{
   List<string> sampleList =
       new List<string>() { "A1", "A2", "A3", "A1", "A2", "A3" };
   sampleList = RemoveDuplicates(sampleList);
   foreach (var item in sampleList)
       Console.WriteLine(item);
}

public class Employee
{
   public int ID { get; set; }
   public string FName { get; set; }
   public int Age { get; set; }
}

public static void TestRemoveDuplicates()
{
   List<Employee> lstEmp = new List<Employee>()
   {
      new  Employee(){ ID=1, Age=20, FName="F1"},
      new  Employee(){ ID=2, Age=21, FName="F2"},
      new  Employee(){ ID=1, Age=20, FName="F1"},
   };

   lstEmp = RemoveDuplicates<Employee>(lstEmp);

   foreach (var item in lstEmp)
       Console.WriteLine(item.FName);
}

public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer);

public class EmployeeComparer : IEqualityComparer<Employee>
{
 public bool Equals(Employee x, Employee y)
 {
   //آیا دقیقا یک وهله هستند؟
   if (Object.ReferenceEquals(x, y)) return true;

   //آیا یکی از وهله‌ها نال است؟
   if (Object.ReferenceEquals(x, null) ||
       Object.ReferenceEquals(y, null))
       return false;

   return x.Age == y.Age && x.FName == y.FName && x.ID == y.ID;
 }

 public int GetHashCode(Employee obj)
 {
   if (Object.ReferenceEquals(obj, null)) return 0;
   int hashTextual = obj.FName == null ? 0 : obj.FName.GetHashCode();
   int hashDigital = obj.Age.GetHashCode();
   return hashTextual ^ hashDigital;
 }
}

public static List<T> RemoveDuplicates<T>(List<T> items, IEqualityComparer<T> comparer)
{
   return (from s in items select s).Distinct(comparer).ToList();
}

public static void TestRemoveDuplicates()
{
   List<Employee> lstEmp = new List<Employee>()
   {
      new  Employee(){ ID=1, Age=20, FName="F1"},
      new  Employee(){ ID=2, Age=21, FName="F2"},
      new  Employee(){ ID=1, Age=20, FName="F1"},
   };

   lstEmp = RemoveDuplicates(lstEmp, new EmployeeComparer());

   foreach (var item in lstEmp)
       Console.WriteLine(item.FName);
}

public static void FindDuplicates()
{
   List<Employee> lstEmp = new List<Employee>()
   {
      new  Employee(){ ID=1, Age=20, FName="F1"},
      new  Employee(){ ID=2, Age=21, FName="F2"},
      new  Employee(){ ID=1, Age=20, FName="F1"},
   };

   var query = from c in lstEmp
               group c by c.Age into g
               where g.Count() > 1
               select new { Age = g.Key, Count = g.Count() };

   foreach (var item in query)
   {
       Console.WriteLine("Age {0} has {1} records", item.Age, item.Count);
   }
}