>>> from ctypes import *
>>> class POINT(Structure):
...     _fields_ = [("x", c_int), ("y", c_int)]
... 
>>> POINT(2, 3, x=4)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: duplicate values for field '???'

The name of conflicting field is not output correctly because it is expected to be bytes.

Proposed patch makes the error message always contain the name of conflicting field.