I simply installed v.2.89 over the "top" of the existing installation, just as I have always done in the past. The previous version was 2.88a. As stated before, the top portion was repeated in the tag panel.
I just now went back and again extracted the zip file that contained the special settings. The tag panel is back to normal now.
Note also that clicking on the mp3tag entry in the context menu that appears when right-clicking on a file now opens the application. I'm not sure if that was a result of reinstalling the special configuration, but it didn't work just after I first installed 2.89.
I've fixed an issue with the latest development build Mp3tag v2.90b, where the configuration file wasn't written if the Window session was closed while Mp3tag was open.
This resulted in Mp3tag performing the Tag Panel update again and produced the duplicate fields.