The point about unique IDs is well taken, but there's a cost to tracking these unique IDs too. For example, if you generate hundreds of message a second, you q

The point about unique IDs is well taken, but there's a cost to tracking these unique IDs too. For example, if you generate hundreds of message a second, you quickly end up having to track millions of unique ids. Then you have to manage the storage of these IDs somehow, so you end up implementing a system to wipe out any ID older than a certain time, and so now you not only have track the IDs you've seen, but when you've seen them. That said, this is probably better than making an incredibly complex message queue system just to support exactly once delivery.