filter distinct items from the array without builtin function - Seeni, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Microsoft (R) Visual C# Compiler version 2.9.0.63208 (958f2354)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        static HashSet<int> dupValues=new HashSet<int>();
        public static void InsertDupVal(int arrVal)
        {
            Program.dupValues.Add(arrVal);
        }
        public static void processDistinctElements(int[] arr)
        {
            int n = arr.Length;
            for (int i = 0; i < n; i++)
            {
                for (int j = i+1; j < n; j++)
                {
                    if (arr[i] == arr[j])
                    {
                        InsertDupVal(arr[j]);
                    }
                }
            }
        }
        public static void Main(string[] args)
        {
            //Write a program to filter distinct items from the array
            //array = [1,4,2,5,4,2,5,1,6,2,5,3,8,9,6]
            //Builtin array functions and LINQ are not allowed.
            
            Console.WriteLine("Start here...");
            int[] nums = { 1, 4, 2, 5, 4, 2, 5, 1, 6, 2, 5, 3, 8, 9, 6 };
            Program.processDistinctElements(nums);
            foreach (var d in Program.dupValues)
            {
                Console.WriteLine(d);
            }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,33 sec, absolute running time: 0,09 sec, cpu time: 0,11 sec, average memory usage: 16 Mb, average nr of threads: 3, absolute service time: 0,45 sec