Update created a duplicate mailbox...?

I just updated my 5,1 Mac Pro to High Sierra after doing a Time Machine transfer of all my data from my old(er) Mac Pro. Mail loaded okay and imported mailboxes. However, all my mailboxes from one provider (Optimum) did not download messages anymore. Seemingly an authentication error.

Then I found (possibly after tinkering with server settings) that Mail created a duplicate mailbox of one of my email addresses. This new mailbox works properly. Now I have two of the same mailboxes... the original has all my old emails and the new one is downloading all newer messages.

What was the cause and how do I remedy this? Is there a way to "merge" the two mailboxes?