I use this script to find identical files with different names on a folder. It is very usefull to when you want to check your media collection for duplicates items. Basically it scans the directory and its subfolders, computes the md5 sum for each file, and keep them in a hash so they can be compared. The buffer size can be bigger or smaller, regarding the amount of memory you want to use. For large directories it can take a while to run, but it worked fine, at least for my 20+GB pictures collection.

#!/usr/local/bin/ruby
require 'digest/md5'
require 'digest/sha1'

$BUF_SIZE = 1024*1024*1024

class Folder_Md5
	
	def initialize(folder)
		@md5_to_files = Hash.new	
		@folder = folder
	end	
		
	def scan
		@md5_to_files.clear
		compute_md5(@folder)
	end		
	
	def md5_for_file(file_path)
		@md5_to_files[file_path]
	end	
	
	def identical_count
		total = 0
		@md5_to_files.each_value do |value|
				if value.size >= 2
					total+= value.size		
				end 
			end
		return total	
	end	
	
	def list_identical
		total = 0
		identities = 0
		puts 'The List of identical files'
		@md5_to_files.each_value do |value|
				if value.size >= 2
					identities+=1
					total+= value.size
					puts 'Idenitical files:'
					value.each{|file_name| puts file_name}
				end 
			end
		puts "got #{identities} identities impling #{total} files"
	end
	
	private	
		def compute_md5(file_path)
			if File.directory?(file_path)
				crt_dir = Dir.new(file_path)
				crt_dir.each do |file_name|
					if file_name != '.' &&  file_name != '..'				
						compute_md5("#{crt_dir.path}#{file_name}")		
					end
				end	
			else
				md5_val = md5(file_path)
				if @md5_to_files[md5_val] == nil
					@md5_to_files[md5_val]  = [file_path]	
				else					
					 @md5_to_files[md5_val] << file_path
				end		
			end
		end
		
		def md5(file_path)
			hasher = Digest::MD5.new
			open(file_path, "r") do |io|
				counter = 0
				while (!io.eof)
					readBuf = io.readpartial($BUF_SIZE)
					putc '.' if ((counter+=1) % 3 == 0)
					hasher.update(readBuf)
				end
			end			
			return hasher.hexdigest
		end	
end

worker = Folder_Md5.new(ARGV[0])
worker.scan
worker.list_identical