Duplicate Finder Lightroom Plugin
The Duplicate Finder plugin uses EXIF data to determine matches between images. That's a useful way around the problem some other methods run into when you get things like "last modified" or even last touched metadata fields in Lightroom's catalog. The plugin is developed by Jim Keir, who is also well known for is very useful Lightroom to Alamy Bridge plugin.
Duplicate Finder installs in Lightroom the usual way you install plugins using the Plug-in Manager.
To use this one you'll need a license directly from the developer. A license costs £ 8.50, and you can buy it here.
Once the plugin is activated with the license and enabled through the Plug-in Manager, you access its functionality by go to Library > Plug-in Extras > Find Duplicates.
The settings are all on a single screen. You can scan the entire catalog, within a selection, or try to find a match just for the selected image/s. You can filter by various EXIF metadata fields and filetypes.
Once you start the scanning, it can take quite a while if you're scanning the entire catalog and have many images in it.
If it finds any, the plugin doesn't actually delete anything. What it does is place the suspected duplicates into a new smart collection called Found Duplicates. From there, you can decide how you want to handle them.
The EXIF comparison method isn't perfect. For one thing, it's quite slow, so be forewarned if you plan to scan a lot of images. For another, it has a limitation in that it can compare capture time only down to the granularity of seconds. When you have rapid-fire shots taken within the same second, they'll come up as false positives unless you're using some other distinguishing criteria in the scan options.
If you come across false positives, you can reset them and remove them from the smart collection by going to the metadata panel, choosing the All Plug-In Metadata view, and deleting the ID number that's in the Duplicate ID field.
Teekesselchen Lightroom Plugin
This plugin is developed by Michael Bungenstock.
Like Duplicate Finder, Teekesselchen is based on comparing EXIF metadata. It offers more granular controls in its settings than Duplicate Finder and, in a feature I find useful, has a nifty way of working around the limitation of differentiating capture times only down to second (more on that below).
The plugin does come with Windows and Mac installers, although I didn't have much luck with the Mac installer. But there's also a Zip file version that's provided for manual installation; that worked better for me. Although the latest version of the plugin is dated 2014, I've found it to work well in Lightroom Classic CC (v.7).
Once the plugin is installed and enabled (use Lightroom's Plug-in Manager to handle that), you access its functionality by going to Library > Plug-in Extras > Teekesselchen > Find Duplicates.
You'll then get the plugin's settings panel. In this instance, I've selected a folder and subfolders that has 556 photos in it, and the scan will run on those.
The Marks and Rules tabs are where you set the parameters of the scan. The Marks tab is where you determine how the results are treated. The Rules tab determines what criteria is used for the matching. One feature I find useful is the ExifTool option; that is a workaround for the limitation of searching only down to a second's granularity that can result in so many false positives.
You can find more detailed explanations of these settings in the documentation that the developer has put together.