double closure

jdee@fatman:~/github/csinitiative/tickdb/control$ irb
irb(main):001:0> f = File.open '/tmp/abc', 'a'
=> #<File:/tmp/abc>
irb(main):002:0> f.close
=> nil
irb(main):003:0> f.close
IOError: closed stream
	from (irb):3:in `close'
	from (irb):3