It reminds me of another old riddle. Looking at a picture, a man says:

"Brothers and sisters have I none,

But this man's father is my father's son."

Who is in the picture?

And, of course, we have to mention the famous song "I'm My Own Grandpaw." This song is difficult even for native English speakers to follow, and it depends on the way we describe family relationships in English. It is probably incorrect--it depends on a verbal trick--the second husband of your grandfather's wife is not really your "grandfather."

Many, many years ago when I was twenty-three

I was married to a widow who was pretty as could be.

This widow had a grown-up daughter who had hair of red.

My father fell in love with her and soon they, too, were wed.

This made my dad my son-in-law and changed my very life

For my daughter was my mother, 'cause she was my father's wife.

To complicate the matter, even though it brought me joy

I soon became the father of a bouncing baby boy.

My little baby then became a brother-in-law to dad

And so became my uncle, though it made me very sad

For if he was my uncle, then that also made him brother

To the widow's grown-up daughter, who, of course, was my step-mother.

My father's wife then had a son who kept them on the run

And he became my grand-child, 'cause he was my daughter's son.

My wife is now my mother's mother, and it makes me blue

Because, although she is my wife, she's my grandmother too.

If my wife is my grandmother, then I am her grandchild

And every time I think of it, it nearly drives me wild

For now I have become the strangest case you ever saw,

As husband of my grandmother, I am my own grandpaw.

There are three people: a grandfather, his son, and the son of the son.  But there are two people who qualify as as a father, and two people who qualify as a son.  So, in effect, you see two fathers and two sons.

You could also count the grandfather as a son, assuming he wasn't formed out of wet mud.  He had to have a father.