asagiと申します。
こんなんで如何でしょうか。

alist = ["Jan","Jan","Jan","Feb","Mar","Apr","May","Dec"]
alist.each do |file|
  open(file, 'a+') do |ofp|
    ofp.puts(file)
  end
end

--
Keiju Aasagi <k.asagi / nifty.com>