In thumbnail view, I only see one image from each group when Hide Grouped Images is selected. However, when going to Slideshow, I suddenly see them all. I'd expect Slideshow to only show those images that are visible in the thumbnail view. In practice, this means that my slideshow shows both the raw and the jpeg version of each image.
This will be fixed in the upcoming 5.6.0 release, refer to https://www.digikam.org/news/2017-05-23-5.6.0-pre-release/ *** This bug has been marked as a duplicate of bug 378552 ***