In Back To The Future II, we learn that traveling forward in time, after having altered the past, lands the traveler into an alternate present or future compared to the one they originally left.
(Thanks to @Xantec for the image)
However, in Back to the Future I, it is clearly implied that Marty's performance of Johnny B. Goode at the Enchantment Under The Sea dance was the inspiration for Chuck Berry's composition of the song.
If, in the altered past, Marty is the inspiration for Johnny B. Goode, then how did Chuck Berry come up with it in the original timeline? Could this be a paradox?