I have tried reverting to When I select Find Duplicates Files from the Tools menu, it opens a box with synchronize.
Tried an earlier version. Tried not using my config .ocb file going back a year or two, no change.
As I have over 100,000 files (in sub folders) under music I would love to genuinely find how many and which are duplicated.
Are you using modified toolbars/menus? It sounds like the menu item to open the Duplicate Files panel is running the wrong command.
You can grab the correct command and drag it to your toolbars/menus via Settings > Customize > Default Toolbars.
If the Synchronize panel is open, you can also switch to the Duplicate Files panel by clicking the panel title, which shows a menu of all utility panel modes.