This is the story of the incredible cloning tax break. In 2004, David Baszucki, fresh off a stint as a radio host in Santa Cruz, Calif., started a tiny video-game company. It was eligible for a tax break that lets investors in small businesses avoid millions of dollars in capital gains taxes if the start-ups hit it big. Today Mr. Baszucki’s company, Roblox, the maker of one of the world’s most popular video-gaming platforms, is valued at about $60 billion. Mr. Baszucki is worth an estimated $7 billion. Yet he and his extended family are reaping big benefits from a tax break aimed at small businesses. Mr. Baszucki and his…