Thanks for the idea. But as MailStores intention is to create a full archive of the email communication which may have to fulfill lawful requirement, we have to consider every new email with a modified header (e.g. different date) as a new one.
I think it's excellent that MailStore prevents duplicate messages from importing. I would love it if MailStore could recognize the latest unique replies in a conversation and eliminate importing all earlier versions of the conversation, unless, for example one of them had something attached to it.