The MQTT 5 (and 3.1.1) specification states:

Until it has received the corresponding PUBREL packet, the receiver MUST acknowledge any subsequent PUBLISH packet with the same Packet Identifier by sending a PUBREC. It MUST NOT cause duplicate messages to be delivered to any onward recipients in this case [MQTT-4.3.3-10].

The broker prevents a duplicate message, but it doesn't response with a PUBREC.