Mailchimp emails show duplicates with (2) and no sender on my iMac

I subscribe to several mailing lists with Mailchimp. With one sender however every message comes in with a (2) beside it . If I click on the down arrow beside the (2) two duplicates then appear, exactly the same message. I have tried to delete them but that does not work. I can delete them on my iphone and iPad where the (2) does not appear but will still be left with the main message on the iMac which them is headed (no sender). This only happens with one Mailchimp mailing list. Any ideas?

[Re-Titled by Moderator]

Original Title: Duplicate Received Mail Messages