The idea is to put one folder to the left and the other to the right pane (see picture), then use a menu command that is usually meant to find differences: Mark > Sync wizard <Ctrl+F9>. The syncrhonization options dialog has a drop down that sets it up for finding Identical files. Click on OK button and let xplorer² select all the files that are identical (duplicates) left and right. Then you can simply delete one set of files.
This comparison will only find duplicates that have the same name. You cannot use it for more complex situations where the names differ. But for a quick and dirty check it is just fantastic!