Finding duplicate integers in an array and display how many times they occurred

Since you can't use LINQ, you can do this with collections and loops instead:

static void Main(string[] args)
{              
    int[] array = { 10, 5, 10, 2, 2, 3, 4, 5, 5, 6, 7, 8, 9, 11, 12, 12 };
    var dict = new Dictionary<int, int>();

    foreach(var value in array)
    {
        if (dict.ContainsKey(value))
            dict[value]++;
        else
            dict[value] = 1;
    }

    foreach(var pair in dict)
        Console.WriteLine("Value {0} occurred {1} times.", pair.Key, pair.Value);
    Console.ReadKey();
}

using linq

int[] values = new []{1,2,3,4,5,4,4,3};

var groups = values.GroupBy(v => v);
foreach(var group in groups)
    Console.WriteLine("Value {0} has {1} items", group.Key, group.Count());