Some of these don't even mind if some are rotated, or cropped and can match color with monochrome versions. On the other hand, there are a significant number of standalone programs that can detect duplicate images based on the actual visual content of the image. In the world of scanning old prints it is not uncommon to find copies of the same images in multiple homes, and even in the same home one may be in a photo album and a copy in a shoe box and the negative in a binder. For example a scan of an 8x10 color print from Mom's attic compared to a slightly cropped 3 x 5 monocrhome printed from the same negative found in Grandpa's basement. A very common situation where metadata is not consistent is scanned images. However these two tools fall apart where metadata is not consistent.

While this works well with images from digital cameras where file names, capture date/time, pixel dimensions and the like are the same on all the dupliate copies. Both of these plugin's only use metadata comparisons to identify potential dupluicates.

These are Teekesselchen and Duplicate Finder 2.

I know of two LrC plugin's that can be used from within LrC for the purpose of finding (and removing) duplicate images.

Duplicate photo fixer pro instructions