using System;
using System.Collections.Generic;
using System.Linq;
 
public class Test
{
	public static void Main()
	{
	    var list = new[] { 80,81,90,90,90,90,90,90,100,85,86,86,79,95,95,95,95 };
	    var dupes = new List<int>();
	    int? prev = null;
	    for(var i = 2; i < list.Length; i++) {
	        if(list[i] == list[i - 1] && list[i] == list[i - 2]) {
	            if(!prev.HasValue || prev.Value != list[i]) {
	                dupes.Add(list[i]);
	                prev = list[i];
	            }
	        }
	    }
		Console.WriteLine(string.Join(", ", dupes.Select(n => n.ToString()).ToArray()));
	}
}

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