Yeah the database ultimately guarantees that. This lets us distribute the fact that a connection exists (in the form of a PID) so that we don't have to go back to the db on every connection request. This helps quite a bit in the case of a lot of connections coming in very quickly, especially when those connections are geographically far from the database or the database is under load from some other system.

If no connection exists yet, you're right, two connections could try to establish themselves at the same time. In which case, whoever gets to the db first wins. But then the loser is still connected to PubSub, so they'll now start getting changes just the same.