uniqifier.rb

require "ffaker"
require 'thread'

class ConcurrentHash
  def initialize
    @reader, @writer = {}, {}
    @lock = Mutex.new
  end

  def [](key)
    @reader[key]
  end

  def []=(key, value)
    @lock.synchronize {
      @writer[key] = value
      @reader, @writer = @writer, @reader
      @writer[key] = value
    }
  end
end


names = []

DupError = Class.new(StandardError)

def unique(values, &block)
  value = nil
  loop do
    begin
      value = yield
      raise DupError if values[value]
      values[value] = true
      break
    rescue DupError
      print "DUP #{value}\n"
      retry
    end
  end
  value
end

cached_values = ConcurrentHash.new

20.times.map do
  Thread.new do
    5000.times do
      names << unique(cached_values) do
        "#{Faker::Name.first_name} #{Faker::Name.last_name}"
      end
    end
  end
end.map(&:join)

puts names.count, names.uniq.count