PhotoStructure’s photo and video deduping, or de-duplication, are a set of heuristics to determine if different files on your computer are actually the same image.
As an example, most digital cameras these days support “shooting RAW,” which encodes images with richer information than JPEG can store. This can mean dramatically better dynamic range, so highlights and shadows can be restored, and more flexibility to adjust color balance. Unfortunately, lots of software doesn't know how to open RAW images, so the option to shoot “RAW+JPEG” is appealing, where each time you push the shutter, both a JPEG and a RAW image is written to your memory card. If PhotoStructure didn't know that these are actually the same image, you'd see two photos with the same image in the PhotoStructure user interface.
PhotoStructure errs on the side of caution, and copies each unique image into your library. If exactly the same image (as in the same stream of bytes on disk) is found, it won't be copied again. All edits to the image will be copied. So in the above case, both the RAW and the JPEG will be copied into your library, the latest-updated version of the image will be shown in the PhotoStructure interface, and if the same exact RAW or JPEG is found in other locations, those files will not be copied into your library, but the path to the image or video will be added to your library's database.
Photo by Erol Ahmed.