It has been observed by implementations that is it possible to replay
a message with a 2nd 5322.From header at the top ...
A thing with two From: headers isn't a valid RFC 5322 message.
You may recall a lengthy argument about what to do with messages with
bare carriage returns, with the final conclusion being "don't do
that." DKIM is only defined to sign valid messages.
If a MUA does something undesirable with invalid messages, I'd encourage
people to improve their MUAs. I expect that an MUA that does something
wacky with extra From: lines also does something wacky with extra Subject:
or other extra lines.
R's,
John
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