Instead we should make a list of old occurrences.
Run through the list of occurrences to be created.
Where an occurrence to be created is on the old list, remove it from the old list, otherwise create the occurrence in the DB.
After all of the new occurrences have been processed, any occurrences on the old list should have their DB entries removed.
Instead we should make a list of old occurrences. Run through the list of occurrences to be created. Where an occurrence to be created is on the old list, remove it from the old list, otherwise create the occurrence in the DB. After all of the new occurrences have been processed, any occurrences on the old list should have their DB entries removed.