Click the Tools icon
and select Photo DeDupe 
.Click the funnel icon
to open QuickFilters.Choose the QuickFilter(s) you’d like to use. For example, select a specific Folder. Click the disclosure arrow
next to By Folder, then navigate to the folder you want to use. This will update the list of suspected duplicates to only show results from the selected folder.Above the results list, click on the Scan Scope dropdown menu. This will let you choose where Photo DeDupe looks for duplicate files.
Entire Library will look at every piece of media in your library to see if there are duplicates of your filtered images.
Within Results will only scan the filtered selection for duplicate files.