array_refinements.rb

module ArrayRefinements
  refine Array do
    def head
      self.first
    end

    def tail
      self[1..-1]
    end
  end
end

clean_photos.rb

require 'photo'
require 'array_refinements'

using ArrayRefinements

dir = '/Volumes/HFS+'
extension = 'CR2'

Dir.glob(File.join(dir, '**', "*.#{extension}"))
  .map { |path| Photo.new(path) }.group_by(&:name)
  .select { |name, group| group.count > 1 }
  .values.each { |group|
    group.head.keep!
  }.each { |group|
    group.tail.reject { |photo| photo == group.head }.map(&:keep!)
  }.flatten.map(&:to_s)
  .tap { |commands| STDOUT.puts commands }

photo.rb

require 'shellwords'
require 'openssl'

class Photo < Struct.new :path
  def name
    File.split(path).last
  end

  def fingerprint
    @fingerprint ||= OpenSSL::Digest::SHA256.new(data)
  end

  def ==(other)
    self.fingerprint == other.fingerprint
  end

  def keep!
    @keep = true
  end

  def keep?
    @keep
  end

  def to_s
    "#{action} #{Shellwords.escape(path)}"
  end

  private

  def data
    File.read(path)
  end

  def action
    keep? ? 'touch' : '   rm'
  end
end