Append behavior of __setitem__

So I'm just starting to read RFC 5322 and I'm starting by skimming  
over Appendix A (differences between RFC 5322 and 2822).  I see this:

26.  No multiple occurrences of fields (except resent and received).*

Which i find very interesting, and possibly relevant to the discussion  
about changing the semantics of Message.__setitem__() to not append to  
the list of headers, as well as some of the other semantics of message  
headers (e.g. get_all()).

thinking-out-loud-ly y'rs,
-Barry