Answer:
Simple. Person A adopted person B as his son...soon to realize he adopted his grandson. How could this happen? Person A's daughter died in a car crash, leaving person B an orphan. Person A was never told that Person B's mother had a son. About 5 years later, Person A decides he wants to adopt a boy. He goes to the local orphanage were all the wild children laid. There, he found Person B, not knowing who he was and what he had been through. Person A adopted Person B as his son and ta da! The mystery's of life.
(This is so informal, but I believe it's enough to answer your question)
"Hoped this helped" :)
Oh and, this triggers a new plot for a new story, thank you!