Short answer: make the event identity durable, deduplicate at the consumer boundary, and resume from a server-issued cursor; a client-side set alone cannot make a marketplace chat room survive reconnects or an incident-response burst. The constraint is trust. A browser reconnects after a laptop...
Source: [Dev.to](https://dev.to/caderaven6851/how-to-scale-realtime-duplicate-event-delivery-nodejs-chat-reconnects-1jbg)