FixConsectiveDuplicates, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // Free all bakery items for free on weekends
            // FREE free engine check and 5 other are live
            // 
            
            string message = "FREE engine engine check check and 5 other other deals deals are deal deal live";                        
            // Console.Write(string.Join(" ", message.ToLower().Split(' ').Distinct()).ToLowerInvariant());
            FixConsectiveDuplicates(message);
        }
        
        public static string FixConsectiveDuplicates(string s)
        {
            StringBuilder sb = new StringBuilder();
            var words = s.ToLower().Split(' ');
            string lastWord = string.Empty;
            foreach(string word in words)
            {
                // Console.WriteLine(word);
                if(!word.Equals(lastWord))
                {
                    sb.Append(word + " ");
                }
                lastWord = word;
            }
            
            Console.WriteLine(sb);
            
            return s;
        }
    }
}

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Compilation time: 0,14 sec, absolute running time: 0,08 sec, cpu time: 0,09 sec, average memory usage: 13 Mb, average nr of threads: 3