using spotlight to find duplicate files

One method:

Enter the file type you are looking for in the search box:

Select the option you require:

then, click on ANY, and select Everything:

The search result has found duplicate files, and duplicate file names:

It would be good idea to quicklook the files you select (Press Spacebar), and check the Info on them (Keyboard: Command - I), before you decide to delete them, as they might be backups, or files copied to different hard drives : if any are connected to your computer when performing the search.