When we build and send emails using MessageBuilder and Email::Sender
we add custom headers defined in SiteSetting.email_custom_headers.
However this was causing errors in cases where the custom headers
defined a header that we already specify in outbound emails (e.g.
the Precedence: list header for topic/post emails).
This commit makes it so we always use the header value defined in Discourse
core if there is a duplicate, discarding the custom header value
from the site setting.
cf. https://meta.discourse.org/t/email-notifications-fail-if-duplicate-headers-exist/222960/14
Martin Brennan