With respect to all of this, I have filed the following feedback with Gmail, Page only knows if they will ever act on it:
Gmail is too aggressive about pruning receipts of duplicate emails. Consider the following scenario:
We have two people, A and B, who both belong to two mailing lists, G and H.
A sends an email to G and H, but B only gets the email from one list, say G.
But what if B wants to respond to the email in the context of list H? B may not even be aware that the email went to H unless they they closely inspect the headers.
I would thus encourage Gmail to consider the tuple (list, person) and not just person when attempting to ascertain whether an email is a duplicate or not.