I still stand to my opinion as you have to compare the files visually - there is no indicator that there may be duplicates for a files - and you do not see only the alleged duplicates but all the files.
There is no dedicated function in MP3tag called "Show duplicates" or something similar.
You are right that it is possible to load several directories.
The way that you described is one way, another would be to drag&drop directories from the Windows Explorer into MP3tag while pressing the Ctrl key.