If Doc's time machine duplicated, why didn't Marty's when he went from 1955 to 1985? Why Doc himself didn't duplicate?
This explanation just doesn't feel BttFy. BttF duplicates things by going into past when the originals are still there. But this time split, it's not even a time split, this time duplication doesn't make sense. And where did the "75 years in the future" come from? The time circuits panel bugged only the 'Destination Time' (resetting it to Jan 1 1885), and the last visited time was still 1985, how and WHY did it duplucate to 2020?