int newNumb= Random.Range(1, 99);
if(acum.Count > 0)
{
    while (acum.Contains(newNumb))
    {
         newNumb= Random.Range(1, 99);
    }
}

static Random _random = new Random();

public static void Shuffle<T>(IList<T> items)
{
    for (int i = thisList.Count - 1; i > 0; i--)
    {
        int j = _random.Next(0, i);
        T tmp = items[i];
        items[i] = items[j];
        items[j] = tmp;
    }
}

var numbers = Enumerable.Range(1,99).ToList();
Shuffle(numbers);
foreach (var number in numbers)
{
     Console.WriteLine(number);
}

Random random = new Random();

    List<int> acum = new List<int>();

    while (acum.Count < 99)
    {
        int Number = random.Next(1, 100);
        if (!acum.Contains(Number))
        {
            acum.Add(Number);
        }
    }

List<int> acum2 = acum.OrderBy(n => n).ToList();

generate list
while list not empty
    choose number from list
    remove it from list
    add to new list

var list = new List<int>();
for (int i = 0; i < 99; i++)
{
    list.Add(i);
}

var resultList = list.OrderBy(i => Guid.NewGuid());

var resultList = Enumerable
    .Range(0, 99)
    .OrderBy(i => Guid.NewGuid());

Random rand = new Random();
HashSet<int> randomHashSet = new HashSet<int>();
while (randomHashSet.Count < 99)
    randomHashSet.Add(rand.Next(1, 100));
List<int> randomList = randomHashSet.ToList();