reject subscription if already initialised

[RichardWright]

We are running a system where subscriptions only need to be initialised. The subsequent new event listeners are just a drag on memory. Currently we place that logic on the client, I'm looking to move this to the subscription server.

1. What does failing to return an async iterable actually do? It's approach I've seen other people use (for example, failing auth)

2. What is a good way to check a subscription has already been created so that subsequent requests can be rejected?

/label help-wanted