reject subscription if already initialised

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

The issue names the subscription server, async iterable handling, and client-side logic, but no file, test, or entry point is identified. First trace how the server creates and retains subscriptions and handles a non-async-iterable result; done requires a defined, tested behavior for rejecting a second request.

Written by the indexing model from the issue text.