Two-headed sharks are now a thing

As reported by National Geographic:

Two-headed sharks may sound like a figment of the big screen, but they exist—and more are turning up worldwide, scientists say... [four examples cited]...
Sans-Coma and colleagues say a genetic disorder seems to be the most plausible cause for the two-headed catshark... But wild sharks' malformations could come from a variety of factors, including viral infections, metabolic disorders, pollution, or a dwindling gene pool due to overfishing, which leads to inbreeding, and thus genetic abnormalities...
Galván-Magaña, who authored the 2011 study, doesn't think two-headed sharks are more common—but rather that there are more scientific journals around to publish accounts.