The day-long battle that begins in book 3 and concludes with book 7 begins and ends also with instances of individual combat: in book 3 it is Paris vs. Menelaos, in book 7 it is Hector vs. Aias, both of which resolve inconclusively. What are we to make of this parallelism?
As a first guess I would put forward that the story told in these books is of how Hector has to take the place of Paris — how Paris’ fault has become Hector’s responsibility.
However, there’s a lot to unpack in contrasting these two duels — in terms of the causes, results, the stakes, the combatants, etc. — and I would just note for now the two most obvious things:
First, that this parallelism exists in the first place and is the sort of thing that establishes the Iliad as an artwork, rather than merely a story.
Second, that it establishes as a sort of “trans-chapter” unit the occurrences of books 3 to 7. I hope to look at the structure and story of that unit a little more closely later on.