In the final analysis ArtScroll translates the discussion on why the torah wrote "and sons shall not be put to death because of fathers." simply that since they wrote at the beginning of the Pasuk that "Fathers shall not be put to death because of sons" it wrote also this statement for "stylistic consistency."
I have see the Gemara explain an extra statement in a Mishna or Beraisa that way (i.e.. "stylistic symmetry"), but a Pasuk in the Torah? Whatever happened to not having any extra words in the Torah?
D Perlysky, Lawrence USA
We do find that the Gemara often tells us (for example, in Bava Metzia 31b) that the Torah speaks the language of the people ("Dibrah Torah k'Lashon Bnei Adam") and uses a double wording, such as "Shaleach Teshalach" -- "you shall surely send away the mother bird" (Devarim 22:7).
We see that sometimes the Torah uses an extra word to help us to understand.
Kol Tuv,
Dovid Bloom