Check that duplicate events are not processed for an art job
Based on stakeholder consent,
art should keep track of which events have been processed for a given
art job. When a duplicate event is encountered,
art will thrown an exception, shutting the process down gracefully.
To be as efficient as possible, sources should be able to specify if they guarantee that they emit only unique events for a given job (e.g.
EmptyEvent). Then such checking can be omitted.