Sum list

sum.cs

public void Calc()
{
  var lists = new List<List<int>>
  {
    new List<int> { 1, 1, 2, 2 },
    new List<int> { 1, 1, 1, 1 },
    new List<int> { 1, 2, 3, 4 },
    new List<int> { 9, 1, 2, 1, 2, 9 }
  };

  foreach (var list in lists)
  {
    var sum = list.Aggregate(new { Sum = 0, LastAdded = list.Last() }, (acc, val) => new {
      Sum = acc.Sum + val - (acc.LastAdded != val ? acc.LastAdded : 0),
      LastAdded = val
    }).Sum;

    // Alternativ
    list.Add(list.First());                                
    var sum2 = list.Zip(list.Skip(1), (a, b) => new { a, b }).Where(a => a.a == a.b).Sum(a => a.a);
                    
    Console.WriteLine($"{sum} == {sum2}");
  }
}