I have $2$ parents, $4$ grandparents, $8$ great grandparents, etc.
So, going back $N$ generations, I have $2^N$ great...great grandparents.
But $2^N$ is seriously divergent. I only have to go back a small number of generations to exceed the total population of the earth!!
That's impossible, so we have a paradox. What went wrong with the calculation?
Some of those great...great grandparents are the same people