According to National Geographic, two-headed sharks are appearing to be discovered with increasing frequency, and scientists aren’t sure why the mutations are happening.

Two-headed sharks may seem like science fiction, but National Geographic is pointing out that they seem to be becoming more common around the world. 
As the report notes, past discoveries include two-headed pre-term sharks found off the Florida coast and in the Indian Ocean as well as two sets of conjoined twins found around Mexico.
Scientists in Spain have also announced that one of their lab shark embryos was growing two heads which is one of the first known instances of the phenomenon appearing in an egg-laying member of the species.  
As the team states in the resulting study, “Each head had a mouth, two eyes, a brain, a notochord [like a spinal cord], and five gill openings on each side.”
They also identified separate hearts, stomachs, and livers but one set of shared intestines, kidneys, and reproductive system. 
While researchers don’t exactly know why these mutations are happening, occurrences in the wild have been attributed to infections, pollution, and overfishing causing the gene pool to shrink. 
A recent BBC report also describes past accounts of two-headed dolphins, snakes, and turtles.