Unique/Overwritable Jobs

[chungwong]

Say we have a queued/scheduled email to be sent out to an email address. The email is somehow part of the input and stored in the underway.task table. If the user changed the email address then it will cause trouble as now a new task needs to be enqueued and the old task needs to be deleted(I am sorry I couldn't come up with a better example/use case).

If would be good if there is an option to make sure the job is unique, on subsequent enqueue on the same task, the last or first one wins, depends on an option. Something similar to ON CONFLICT in the SQL world.