My grandfather and their great-grandfather were the same person. How are we related?

Thanks

Parent/child

2nd cousins for instance my daughter has a 1st cousin named A. My daughter and A share the same grandfather. A had a child named B so B is now the great grandchild of my daughters grandfather. And B is my child's 2nd cousin and grandpas great grandchild.

Ernest was my grandfather. He is my children's great-grandfather. (me as parent & my child)
Ernest was my siblings' grandfather. He is their children's great-grandfather. (my sib & my niece or nephew)
Ernst was my first cousins' grandfather. He is their children's great-grandfather. (my first cousin & my first cousins once removed)

2nd cousin, niece or nephew, or son or daughter