If you have duplicates that do not appear in the list then you will need to use ftp or whatever other tools you would normally use to access your server files.
An SP plugin will normally show an ‘uninstall’ option if there is actually anything to remove from the database i.e., data associated with the plugin or new columns or database tables. If the plugin has no such data then there is nothing to uninstall. Like standard WordPress plugins, deactivating them effectively removes them from being used.
The forthcoming release – 5.2.0 – does, as it happens – add the option to delete them as well.