I doubt that it is possible to check for plagiarism automatically because in the social network itself it happens regularly. Therefore, it all depends on the diligent work of the manager.
Yes, I am sure he did well and carefully. For campaign that limit the number of participant, I think the manager can still check all report carefully manually. But I would highly doubt a campaign that has no participant restriction, it will make it difficult for manager to check. If campaign rules are tightened, this will probably prevent more such case in the future. Now, managers have enforced stricter rules about it.