I didn’t notice that a document was already open and clicked on it in the main file list.
That opened a second copy of the doc.
In such a case, it would be best to have a screen prompt that the doc is already open, just to make sure the user really wants a second copy of it to be opened.
Mac OS 10.14.3 (Mojave)
Only Office v. 8.0.1
Desktop Editors / Free Edition