Find duplicates in an array in C#

Solution 1:

// Rearrange array in alternating positive
// & negative items with O(1) extra space
using System;

namespace ConsoleApp1
{
    internal class Program
    {
        static void Main(string[] args)
        {
            int[] arr = { 1, 5, 7, -1, 5 };
            List<int> list = new List<int>();
            StringBuilder stringBuilder = new StringBuilder();
            stringBuilder.Append("Duplicate values: ");
            for (int i = 0; i < arr.Length; i++)
            {
                int duplicay = arr.Count(x => x == arr[i]);
                if (duplicay > 1)
                {
                    if (list.Count(x => x == arr[i]) < 1)
                    {
                        list.Add(arr[i]);
                        stringBuilder.Append(arr[i]+",");
                    }
                }               
            }
            Console.WriteLine(stringBuilder.ToString());
            Console.ReadKey();
        }
    }
}