using System;
using System.Collections.Generic;
using System.IO;

public static class ClipboardFusionHelper
{
    public static string ProcessText(string text)
    {
        var file = @"c:\test.txt";

        if (!File.Exists(file))
            File.Create(file);

        if (File.ReadAllText(file).Contains(text))
            return null;

        File.AppendAllText(file, text + Environment.NewLine);
        return null;
    }
}