Mail -- how to automatically delete duplicate imported messages

Got a question regarding importing -- can you only just import mailboxes or individual messages? When I've always tried an import, i only seem to be able to import mailboxes. I have a in iMac and a MB pro, and right now the only way I seem to be able to keep the mailboxes in either one current and up to date with the mailboxes on the other machine is to do an import, then take the messages from the imported boxes and move them to the appropriate folder; but this presents a problem: a lot of times the imported boxes will have a lot of the some messages as the box in Mail -- which means that I've got a mailbox with duplicate messages on them. Is there any way for mail to recognize duplicate messages and delete one of them? Or what is the most efficient way to take care of this problem, other than going through each imported mailbox and manually deleting the duplicate messages?

Or what is the most efficient way to take care of this problem, other than going through each imported mailbox and manually deleting the duplicate messages?

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Which mail protocol are you currently using with your ISP? If you're using PoP mail, switch to IMAP if your ISP has it available.