Apple TV Photo Slideshow Repeats Photos Over and Ove

• The slideshow is supposed to show all compatible images in the current album, then repeat. This would be directly from the source (iCloud Photos or perhaps Home Sharing), so not limited for large albums.
• The screen saver from a photo album is slightly different, in that it is limited in capacity (not sure if that is by total media file sizes, or a number of items) as the images are copied locally, so it may show a subset of larger albums, and then repeats. I would suggest to create smaller albums, and alter between album when you feel you’ve seen enough of it.

I think if you're running the slideshow for a certain album, the slide duration must be long enough for the next picture to load (e.g. 5 or 10 seconds). If there is not enough time, Apple TV will back track to an earlier slide and go forward again when the batch of newer slides finish loading. I use Verizon FIOS and the 2sec duration always causes repeats, 5 sec has a lot less repeats for 4k photos.