Apple Mail: IMAP account - message has exactly 2 messages, cannot swipe-to-delete

When using Apple Mail with a Zimbra IMAP server occasionally a message will come in and will show as (2) messages. When expanding the messages each message is an exact copy - same timestamp and all. This is regardless of if the email is plain text or HTML.

This behavior is NOT present in the webmail interface.

If the message is from a list or something I can easily ignore I typically do the two-finger “swipe-left” at which point the entire message preview turns red and the trash icon with (2) is all I can see on the left side.

If I click on the messages and click the Trash icon one message vanishes and the other is deleted. But the other deleted emails appear greyed out above them.

I have tried deleting the account and re-adding it but this does not work.

Is anyone else having this problem?