Who cares if the computer takes a few more cycles to execute something? It does matter if it takes a programmer a week to make a change that would have taken only an hour if she had understood your code.

    numbers = []
    series.quantity.to_i.times do
      begin
        number = rand 99_999
        raise if number < 10_000 or numbers.include? number
        numbers.push number
      rescue
        retry
      end
    end

    numbers.each do |number|
      begin
        pin = rand 999_999
        raise if pin < 100_000
      rescue
        retry
      end

#    skip...
    end

    numbers = {}
    while series.quantity > numbers.length do
      number = rand( 89_999 ) + 10_000
      next if numbers[number]
      numbers[number] = 1
      pin = rand( 899_999 ) + 100_000
# практика телесных наказаний должна быть возрождена
#   skip...
    end