During startup, is_valid_fd() in Python/pylifecycle.c is called to validate stdin, stdout, and stderr. Performance isn't critical here, but every bit helps in reducing startup time. In my tests, implementing this check in Windows via GetFileType((HANDLE)_get_osfhandle(fd)) is 5-6 times faster than close(dup(fd)). For example:

#if defined(MS_WINDOWS)
    HANDLE hfile;
    _Py_BEGIN_SUPPRESS_IPH
    hfile = (HANDLE)_get_osfhandle(fd);
    _Py_END_SUPPRESS_IPH
    return (hfile != INVALID_HANDLE_VALUE &&
              GetFileType(hfile) != FILE_TYPE_UNKNOWN);
#endif