Improve duplicate message handling

[elizagamedev]

notmuch combines all duplicates it detects into a single virtual message entry. If notmuch detects such duplicates among JMAP emails, the "source of truth" for which JMAP email entry maps to the local notmuch message is non-deterministic. When pushing changes, all remote duplicate messages are updated to match.

Maybe the best course of action would be not to store duplicate messages in the first place. Fastmail has a duplicate message detector which can help find and delete these problematic messages. That said, mujmap could come up with a scheme to handle these cases more robustly.