Clients can choose to ignore multiple deliveries of the same message. They keep the message id around until they get an acknowledgement from the server that it

Clients can choose to ignore multiple deliveries of the same message. They keep the message id around until they get an acknowledgement from the server that it won't be sending any more of that message. This is part of the protocol. What it doesn't really guarantee is the order, which requires even more clever programming and more network exchanges.