traceback when option is in config file more than once

E.g. when I have two "debug_flags" lines in my configuration file, I get:

Traceback (most recent call last):
  File "./bzr", line 145, in <module>
    exit_val = bzrlib.commands.main()
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/commands.py", line 1212, in main
    _register_builtin_commands()
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/commands.py", line 171, in _register_builtin_commands
    import bzrlib.builtins
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/builtins.py", line 21, in <module>
    import bzrlib.bzrdir
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/bzrdir.py", line 2154, in <module>
    format_name = config.GlobalStack().get('default_format')
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/config.py", line 3153, in get
    expand = _get_expand_default_value()
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/config.py", line 188, in _get_expand_default_value
    expand = conf.get_user_option_as_bool('bzr.config.expand', expand=True)
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/config.py", line 397, in get_user_option_as_bool
    s = self.get_user_option(option_name, expand=expand)
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/config.py", line 375, in get_user_option
    value = self._get_user_option(option_name)
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/config.py", line 887, in _get_user_option
    value = self._get_parser().get_value(section, option_name)
  File "/home/jelmer/src/bzr/bzr.dev/bzrlib/config.py", line 777, in _get_parser
    raise errors.ParseConfigError(e.errors, e.config.filename)
bzrlib.errors.ParseConfigError: Error(s) parsing config file /home/jelmer/.bazaar/bazaar.conf:
Duplicate keyword name at line 27.