Some queues offer "at-least-once" delivery (such as SQS) meaning even if you do ack/delete the message, you can still get it again. IronMQ will not deliver a me

Some queues offer "at-least-once" delivery (such as SQS) meaning even if you do ack/delete the message, you can still get it again. IronMQ will not deliver a message again if it's been acked/deleted. And yes, work should be idempotent, as is the case with any queue you use.