It's infuriating how the client must be stateful and have local storage, for both the access_token and the last message recieved. That's right you must remember as the client where the last events [1] you've seen (even if you already told the server to mark it as read) was or else the server will happily send you the same messages over and over again across restarts of your client.
I kind of miss making IRC bots where things were much simpler and ... quicker honestly (latency wise).
[1] https://uhoreg.gitlab.io/matrix-tutorial/sync.html#:~:text=w...