 One initial idea for the sequel had the first two acts the same, but  the third act had Biff giving his younger self the almanac in the  1960s, not in 1955. When Marty went back to stop him, he ran into  his parents again (who were now hippies) and almost stopped his own  conception. Robert Zemeckis (qv) decided that it was too similar  an idea to the first film and came up with the idea of revisiting  the original film from different angles with the two Martys in 1955.