Please note that the concept of middle name is unknown in the Netherlands. That is why we always check the "No middle name" field in Dutch profiles. If more then one first name is present we record them all in the "Proper First Name" field.
What you interpreted as middle names of the persons related to this marriage are actually patronymics. Starting in 1811 Dutch people had to register a surname. Before that people mostly used the patronymic, so Kornelis would have been named just Kornelis Pieters. For reasons unknown to me in Friesland they often continued the (unnecessary) use of patronymics, combined with using surnames, as in in your example.
