It is one of the hardest parts of preparing the data. The primary responsibility for being on the list is with the entrants - they are asked to confirm previous finishers on the entry submission. Some people use this field even when they have not finished before.
There are also quite a few people who are previous finishers but don't remind the organisers and by my calculations the following really are previous finishers:
|James Andrew Collister|
|Nour Addine Ayyoub|
|Paul C Du Toit|
If it was someone else with the same name please email firstname.lastname@example.org to correct me.