code: def from_flag(cls, flagname, flag_values, other_flag_values=None):
      
      first_module = flag_values.find_module_defining_flag(
          flagname, default='<unknown>')
      if other_flag_values is None:
        second_module = _helpers.get_calling_module()
      else:
        second_module = other_flag_values.find_module_defining_flag(
            flagname, default='<unknown>')
      flag_summary = flag_values[flagname].help
      msg = ("The flag '%s' is defined twice. First from %s, Second from %s.  "
             "Description from first occurrence: %s") % (
                 flagname, first_module, second_module, flag_summary)
      return cls(msg)
docs: Creates a DuplicateFlagError by providing flag name and values.
  
      Args:
        flagname: str, the name of the flag being redefined.
        flag_values: FlagValues, the FlagValues instance containing the first
            definition of flagname.
        other_flag_values: FlagValues, if it is not None, it should be the
            FlagValues object where the second definition of flagname occurs.
            If it is None, we assume that we're being called when attempting
            to create the flag a second time, and we use the module calling
            this one as the source of the second definition.
  
      Returns:
        An instance of DuplicateFlagError.