It’s actually a common one. They share similar but opposite backstories:
Both are born from fathers who run major miltiaries in the world and then abandoned by them because they’re a weakness.
Both were constantly beat up by their older siblings and teased for being different
Both were then adopted and taught their morals from a seasoned pirate who got dismembered saving their lives (Shanks an Arm and Zeff a Leg)
Both had a dream to find a legendary treasure they were laughed at for believing (A sea with all the ocean has the offer and a treasure with all the world has to offer)