I've committed Berker's patch from #27331, and I'm about to take the new email API out of provisional status.

Barry is committed to not changing this behavior in 2.7 and I agree.  In any case 2.7 doesn't differentiate between headers being added by the user and headers coming from the parsed message, and the latter *have* to allow duplicates even if the fields aren't supposed to be.  The python3 code does make a distinction between these two cases, which is what allowed me to do the fix in the new policies.  (Yes, you *could* fix feedparser and message in 2.7 so it also could tell, but that is an invasive change).

So, I'm re-closing this as fixed.  It's "won't fix" for 2.7 and compat32.