BEIJING—China is poised to overtake the U.S. as the biggest movie market in the world, but how many people are really in the audience?

After “Ip Man 3”—a Chinese martial-arts movie in which Mike Tyson plays a crooked property developer—pulled in 470 million yuan, or $72 million, over its opening weekend, Chinese regulators said this week they are investigating whether the distributor tried to boost the movie’s box office by buying tickets for “ghost screenings.”

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