A good way to visualize this is to imagine a family tree going back for many, many generations. You'd find that certain males were your ancestor "more than once

A good way to visualize this is to imagine a family tree going back for many, many generations. You'd find that certain males were your ancestor "more than once", i.e. you might find the same man was your ancestor on both your mother and father's side. Of course, females can also be your ancestor "more than once"; it's just that, as a matter of fact, more men have this property. This is due to the fact that some men have dozens of children, by several different women.