<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def from_flag(cls, flagname, flag_values, other_flag_values=None):
 """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. """

    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)