Mine removes duplicate headers where the field value can't be a list.
E.g. multiple From, To, Cc are joined into one with all the values.
I often receive mail from VAX/VMS systems with From fields like
Joe Bloggs, Room 10, Tel. extn 203 <BLOGGS(_at_)SCHOOL(_dot_)EDU>
The mailer on the system I use (Edinburgh University's own operating system,
own mailer, UK Grey Book, high quality) tries to reply to three "people"
Tel. extn 203 <BLOGGS(_at_)SCHOOL(_dot_)EDU>
and fails with the first two.