using System;

using System.Collections.Generic;

using System.Linq;

public static class Utils

{

    public static IEnumerable<T> UniqueConsecutive<T>(this IEnumerable<T> input)

    {

        bool first = true;

        T current = default;

        foreach(var element in input)

        {

            if(first || !element.Equals(current))

            {

                yield return element;

                current = element;

            }

            first = false;

        }

    }

}

​

public class Program

{

    public static IEnumerable<int> Sequence()

    {

        yield return 1;

        yield return 1;

        yield return 1;

        yield return 1;

        yield return 2;

        yield return 3;

        yield return 3;

        yield return 7;

        yield return 8;

        yield return 9;

        yield return 10;

        while(true)

        {

            yield return 20;

        }

    }

    public static void Main()

    {

        foreach(var e in Sequence().UniqueConsecutive().Take(5))

        {

            Console.WriteLine(e);

        }

    }

}