This morning in Shul a fellow asked me an interesting question regarding Tefilin. Both the Tefilin Shel Yad & Tefilin Shel Rosh have the same 4 Parshiyos written in them. What is the purpose of having these same 4 Parshiyos twice? Why not divide it between the 2 or why not have different Parshiyos in the 2 Tefilins? In other words, what is the purpose of the duplication of Parshiyos in the Tefilin?
I feel like your friend may be overthinking this.
וקשרתם לאות על ידך והיו לטטפת בין עיניך
We are commanded to "tie them for a sign upon your hand and they should be as a badge between your eyes". The verse says "and" which means you need to put them in both places.