using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		var a = @"\server\MainDirectory\SubDirectoryA\RN012345\SubdirectoryC\MyFile.pdf";
		//var a = @"\server\MainDirectory\SubDirectoryA\RN111222\SubdirectoryC\MyFile.pdf"; // NO REPLACE
		var res = Regex.Replace(a, @"\\RN([0-9]{6})\\", m =>
				m.Groups[1].Value.Distinct().Count() == m.Groups[1].Value.Length ?
				  "\\RN0123456\\" : m.Value);
		Console.WriteLine(res);
	}
}

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