Open FSlint. First lets remove duplicate files. Add a folder or folders you want to clean from duplicate files. And then just click Find. Do it for the personal files and folders, better not touch system files and folders. After you found the duplicates you can select the ones you want to remove and hit delete.
Meld has a bit different approach. First you need to choose between file and directory comparison. At that point click directory comparison. Next you need to line out the folders you want to compare. Then you need to tell the application to show identical (same) files in the selected folders. If the result is empty then you dont have duplicated files in those folders. Those files that appear are then duplicates.
dupeGURU is pretty simple to use. From the bottom of the app window just hit + button and select the folders you want to search. Then hit the scan button. The result will appear in a new window where you have many actions to choose from.