It's not very easy to find duplicates in Lightroom as you might have photos in your Catalog that are different but share the same filename or you may have several copies of the same photo with different file extensions or filenames. Here's a method of locating duplicates in Lightroom Classic:
1. Go to the Catalog panel in the Library module and click on the All Photographs Collection. This selects all the photos in your Catalog.
2. Click G to go to Grid View (if you’re not there already). If you can’t see the Filter Bar press the \ key to display it at the top of Grid View.
3. To carry out the search, click Text and select Filename and Contains All (see below). Type the name of the file you’re searching for into the search bar. You can omit the file extension to reveal more versions of the photo.
The search reveals all photos that share the filename entered. You can enter a partial filename instead of a full one if that helps your search.