int[] arr = { 1, 16, 3, 37, 66, 5, 66, 7, 55, 101, 11, 22, 8, 25 };
int?[] idx = new int?[101];
Tuple<int, int> t = null;
for (int i = 0; i < arr.Length; i++)
{
if (idx[arr[i]].HasValue)
{
t = Tuple.Create(idx[arr[i]].Value, i);
break;
}
idx[arr[i]] = i;
Console.WriteLine(t == null ? "Нет такого числа" : $"{t} = {arr[t.Item1]}"); ;
}
Console.ReadKey();
.