I just realised this happens a lot in my family:
My grandfather was named James, but everyone called him Jim.
My uncle is also named James, everyone calls him Jimmy.
My cousin (his nephew), surprise surprise, James, but we all call him Jamie (even though he insists on being called James now).
My great-uncle is called Paul, but we call him Duck (because it's far more polite than another word it rhymes with).
My father is Paul as well, but beyond a pub nickname of "Black Paul" (which is used if Liverpool ever lose), you can't really shorten it.
Then there's me, and because of my father, most of our extended family calls me Paul Joseph to differentiate us, which has kind of gotten to friends so that's what most people call me nowadays. Used to hate it when I was younger, but meh, it's not too bad these days.