+ * Whether the queue should reject insertion of this job if a duplicate exists
+ *
+ * This can be used to avoid duplicated effort or combined with delayed jobs to
+ * coalesce updates into larger batches. Claimed jobs are never treated as
+ * duplicates of new jobs, and some queues may allow a few duplicates due to
+ * network partitions and fail-over. Thus, additional locking is needed to
+ * enforce mutual exclusion if this is really needed.
+ *
+ * @return bool