Actual Over The Air update you receive and install automatically, or sideloading?
Actual Over The Air automatic update definitely flashes to both slots. The nature of how two slots works is the OTA updates the slot you're not using first, then when you reboot you're booting into the newly updated slot, and then the OTA applies to the slow you're no longer using.
I don't sideload so I have no idea if there's any difference in behavior. Plus some people haven't had success with sideloading the OTA and have to use another method (either full factory image and/or the Official Google Android Flash Tool.