When rendering, the status bar shows building BVH with a percentage, and “duplicates” with a percentage.
What are the duplicates in this context?
I think this means you have Use Spatial Splits checked in Cycles Render->Performance->Acceleration Structure, and it refers to the additional BVH nodes that are generated by “splitting” existing BVH structures as part of that process.
But that’s mostly a guess based on looking at code that I’ve never looked at before 
1 Like
Ahh - you are on to something. I have a particular scene that just hangs (not crashes, the timer is still going, it just doesn’t progress beyond the building BVH stage). Turning off spatial splits and it whizzed through.