2.1.2.1 Dup reads in part:

*****
If Dup is 0 then the flow is the first occasion that the Client or Server has attempted to send the MQTT PUBLISH Packet.
....
The Dup flag MUST be set to 1 by the Client or Server when it attempts to re-deliver a PUBLISH Packet
*****

For the first sentence, "...sent the MQTT PUBLISH Packet." should read: "...send a MQTT PUBLISH Packet." Yes?

That is the same usage you see in the second sentence.

The difference being that "a MQTT..." applies to any MQTT... whereas "the MQTT..." means some packet in particular.

I haven't scanned to the document for all uses of the definite article and doubt I will have time. Just a word to alert you to the issue, which you already handle correctly in many other cases.