You don’t really “run it twice,” but if you are updating from a very old version, you will notice that it seems like it does multiple updates.
Look in the list of files… you’ll see two system packages. One is for the new version (16.0), and the other one is for the version released in November 2020 (14.10, the first version with profiles and the new home screen). Basically, if you’re running anything older than 14.10, it will increment to 14.10, then it will turn around and do the latest update. If you are running 14.10 or newer, it’ll just install the latest update.
I ran into this when I updated my wife’s
Palisade, which was running 14.5. It ran through the entire update process to get it to 14.10, seemed like it was done, and then immediately started updating again to get to 16.0. So it took about twice as long as it took me to update my Telluride from 15.5 to 16.0, because it essentially did two updates.
As I mentioned earlier, the list of available options for updating depends on what you’re coming from. They only recently began offering the “update after vehicle is turned off” option. So if you’re coming from anything other than a recent version, you won’t see that. So you’ll have to either run the car or keep close eye in accessory mode. It pays to keep the car up to date for this reason.