using System;

public class Test
{
    public static void Main()
    {
        int?[] arr = new int?[10] { 1, 1, 2, 3, 2, 1, 10, 10, 4, 3 };
        for (int i = 0; i < arr.Length - 1; ++i)
        {
            if (arr[i] == null)
            {
                continue;
            }
            int count = 1;
            for (int j = i + 1; j < arr.Length; ++j)
            {
                if (arr[i] == arr[j] && arr[j] != null)
                {
                    ++count;
                    arr[j] = null;
                }
            }
            if (count >= 2)
            {
                Console.WriteLine(arr[i]);
            }
            arr[i] = null;
        }
    }
}