ListDuplicateFiles.cs

/// <summary>
/// Processes a given directory, grouping and listing files with duplicate content
/// </summary>
/// <param name="directory">The path to process</param>
private void ListDuplicateFiles(string directory)
{
    // Calculate and store the hash and path for each file in the directory
    var files = Directory.GetFiles(directory).Select(f => new { Path = f, Hash = FileHash.CalculateFromFile(f) });

    // Group and iterate when duplicates exist
    foreach (var group in files.GroupBy(f => f.Hash).Where(g => g.Count() > 1))
    {
        Console.WriteLine("--- Dupe ({0})-------------------------------", group.Key);
        foreach (var file in group)
        {
            // List each file
            Console.WriteLine("\t  {0}", Path.GetFileName(file.Path));
        }
    }
}

Supporting-FileHash.cs

public static class FileHash
{
    /// <summary>
    /// Generate an MD5 hash of the given file
    /// </summary>
    /// <param name="filename">The content file</param>
    /// <returns>The generated hash</returns>
    public static string CalculateFromFile(string filename)
    {
        // read file bytes
        byte[] bytes = File.ReadAllBytes(filename);

        // md5 calculation
        MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
        byte[] output = md5.ComputeHash(bytes);
        string hash = Convert.ToBase64String(output);

        // returning
        return hash;
    }
}