There are several products that can compare the actual content of the image (pixels) to ferret out probably duplicates. These tend to work outside of LrC but some can use the LrC catalog as a source for images to look at. This class of product create a low res, usually monocrhrome, version of the images and then compares those. some will detect rotated images and images with different cropping. In most you can tell it what criteria to use and in what order as well as how close they need to match in order to be flagged as potential duplicats. They each of course have different designs and some are better than others in certain areas.
For scanned images the file name, pixel count, files size, or any metadata is useless so for your purpose you would turn off those search criteria and just leave the Pixel compare. For a few thousand images they seem to work OK but for tens of thousands of images they tend to bog down to a crawl.
You may want to look at Duplicate Photos Fixer Pro as an example but there are many others.