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
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