only know 34 of my 2048 9x great-grandparents, and 6 of those 34 are the same 2 people. Another set of 9x great grandparents are also 10x great-grandparents of mine. Going back another coupla generations and John Lothrop (1584 - 1663) is both my 12x great-grandfather through a son of his first wife and my 11x great-grandfather through a son of his second
those 5 people = 12 of my ancestors. Not quite as confusing or silly as that I'm My Own Grandpaw song, but nonetheless a pretty awesome display of inbreeding

Alfonso XII is even cooler though. He wins for being the most inbred person ever. Dude's grandpas were brothers and his grandmas were sisters. That means that instead of the usual 8 great-grandparents, he only had 4. Only having half the usual number of ancestors 3 generations back = inbred to the extreme, but Alfonso Francisco de Asís Fernando Pío Juan María de la Concepción Gregorio Pelayo was even cooler than that because his grandfather brothers didn't just just marry any two sisters, they married the daughters of their sister. That's right, married their nieces. King Carlos IV and his wife (who was also a first cousin) married two of their sons to two of their granddaughters. Plenty more cousins marrying cousins, but it makes the brain hurt trying to understand everything
scroll down to the section entitled Ancestry and click on the blue [show] link in the box that says Ancestors of Alfonso XII of Spain to witness the insanity.