C#: How do you read a text file into a array removing dupilcate lines from a Second Log file

Hi

I have a list of URL's in a file that I need to process
and a List of  finished URL's in another Log file
after I've finished with a URL I write / append it to the Log  File

How do check the current URL isn't in the Log File therefore not already processed

using System;
using System.IO;
using System.Linq;

public class Program
{
	public static void Main()
	{
		    string LogFile = System.Environment.CurrentDirectory + "\\Log.txt";
            var URLFromLog = File.ReadAllLines(LogFile).Select(l => l.Trim());
            var URLFromURLs = File.ReadAllLines(@"Path\To\URLs.txt").Select(l => l.Trim()); // This File may contain Duplicates and changes each time the program runs
		// Only Add to URLs if unique and not already in the Log File

// This doesn't look right but unsure what it should be
            var URLs = URLFromLog.Distinct().Concat(URLFromURLs).ToList();
		foreach (string sURL in URLs)
		{
			    var URL = sURL;
			// Do Stuff with URL 
                File.AppendAllText(LogFile, sURL + Environment.NewLine);
		}
	}
}

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