Video Comparer is a commercial tool which can find similar videos by their content.
The important word here is "similar". The program doesn't just look for identical files. It can also match files when one has been cropped, rotated or resized. It's even able to identify videos where one clip exists within another (video A is a single scene extracted from video B).
Video Comparer works much like any other file comparison tool. Point it at the folder tree you'd like to examine, perhaps choose an option or two (there are four comparison modes, for example, from quick to full), and click Scan. The program crawls your folders, looking for videos (avi, mpg, mpeg, mkv, wmv, 3g2, 3gp, asf, asx, bsf, divx, f4v, flv, m2p, m2v, m4v, mov, mp4, ogv, rm, ts, vob and xvid files are supported).
The comparison process is surprisingly quick. The developer quotes 1 minute for 30 videos, 5 minutes for 100 videos or 40 minutes for 500 videos, and we wouldn't disagree.
Once Video Comparer has finished, it displays any likely duplicates. Synchronised timeline thumbnails show you the matching scenes, so you can see at a glance if the program is correct, and true duplicates may then be copied, moved or deleted with a click.
Please note, the trial version compares files only. It won't allow you to delete or back up duplicates, and it doesn't display the names of the files.