using System;

using System.Linq;

​

namespace ConsoleApp

{

    public class Program

    {

        public static void Main(string[] args)

        {

            var buzzword = "buzzword";

            var lineToAdd = "Line to add";

            var lines = new[]

            {

                "Line 1",

                "Line 2",

                "Line 3 contains the buzzword",

                "Line 4",

                "Line 5 has the buzzword too and is already followed by one line to add",

                "Line to add",

                "Line 6 with the buzzword is already followed by two lines to add",

                "Line to add",

                "Line to add",

                "Line 7",

                "Line 8 with the buzzword"

            };

​

            var transformed = lines.SelectMany((line, index) =>

                line.Contains(buzzword) && (index == lines.Length - 1 || !lines[index + 1].Equals(lineToAdd, StringComparison.Ordinal)) ?

                new[] { line, lineToAdd } :

                new[] { line })

                .ToArray();

​

            foreach (var line in transformed)

                Console.WriteLine(line);

        }

    }

}

​