What is a Janus Cat?
Every so often a rare kitten is born with two faces. This condition, known as diprosopus, is one that can also affect humans and other types of animals, such as sheep, dogs and pigs. There is even documentation it happens to snakes. Cats born with diprosopus are also called "Janus cats".
An Overview of Diprosopus
Diprosopus is a rare congenital defect that is also known as craniofacial duplication. The condition refers to a fetus with one trunk, normal limbs, but has facial features that “are duplicated to a certain degree”, notes The Embryo Project Encyclopedia. 
How this duplication happens can vary. When this birth defect is present, some born with it have all of the face duplicated, others have only a partial duplication. Some born with diprosopus are not born with brains, others have neural tube defects or heart malformations. Diprosopus is believed to be caused by too much presence of a protein called sonic hedgehog homolog, often referred to as SHH. While some information is known about Janus cats, there is still a lot of mystery surrounding this unusual condition.
This photo is of Frank and Louie, a Janus cat who holds the record for the longest living of its type (more about Frank and Louie later in the article). Frank and Louie lived to be an amazing 15 years old.
If not stillborn, most born with diprosopus do not have a very long life expectancy; a large percentage of them do not make it past infancy and/or live long beyond birth. There is no treatment for diprosopus. In cats, it is not unusual for their mothers to abandon them.
Deucy was a Janus kitten born in June 2013. Her mother had abandoned her at birth, which is a common thing for adult cats to do when a kitten is born with diprosopus.
Why Are These Cats Named ‘Janus’?
Janus cats are named after the two-faced Roman God. Janus is an ancient God that represents new beginnings, with one face looking to the future, the other, to the past.  January is named after this god and his image can also often be seen on gates and doors. This god was also highlighted twice in the 2010 movie ‘The Tourist”, which starred Johnny Depp and Angelina Jolie.
“The Roman god, Janus. My mother gave it to me when I was little. She wanted to teach me that people have two sides. A good side, a bad side, a past, a future. And that we must embrace both in someone we love....,” said Elise, who was the character played by Jolie, and wore a bracelet with Janus as a charm.
Janus also appeared later in the film during one of the flick’s climatic end scenes. The god had appeared as a wall hanging that covered a safe.
Scene where Elise, Jolie's character was describing Janus.
In recent years, there have been several news reports about these rare Janus cats.
Well-Known Janus Cats in Recent Years
I first learned of a Janus cat about five years ago when I read about a lovable cat that had just turned 12 years old. As I became interested in these rare cats, I discovered there have been a few more born in subsequent years.
Frank and Louie
Frank and Louie was designated the honor as being the oldest living Janus cat at the age of 12 in the 2012 Guinness Book of World Records; he reached this milestone in 2011. He was born with one brain, two noses and two functioning eyes. Frank and Louie also had a center eye, which was blind. He sadly passed away in December 2014, having reached the remarkable age of 15.
His owner, Martha "Marty" Stevens, had cared for him since birth, having saved him from being euthanized when he was a day old; she tube-fed him for his first three months of life. Over the years Frank and Louie became very well-known and was loved all over the world.
[ Related Reading: Fostering Kittens or Cats ]
Harvey Dent was born in Port Charlotte, Florida in 2012. This kitty was named after the two-faced character from “Batman”. He was born with four eyes, two mouths and two noses. All appeared to work normally at the time of birth. There were initial hopes Harvey Dent would make it, but sadly he died two days after he was born.
News story about Harvey Dent
Deucy was a female born in June 2013. She was born with one body and two faces, with one being able to meow. She was rejected by her mother, and her owner, Stephanie Durkee, fed her every two hours by using a syringe. Durkee had hoped that Deucy would be one of those rare few Janus cats that live long and happy lives. At the time of her birth, she was described as being the strongest cat in the litter. Unfortunately, Deucy only lived a couple of days.
Video of Deucy in June 2013. Sadly, despite the strength she showed at birth, she only lived two days and died of complications relating to diprosopus.
Dragon was born in March 2016 in Voronezh, which is located in south-western Russia. This kitten has two completely formed faces, one was described as not being able to move when it was born. The owners said she was very weak at birth. I looked for updated news on this kitten, but haven’t found anything. Did come across one YouTube link that was posted around the time of Dragon's birth.
There are some other vague reports of Janus cats dating back to the first half of the 20th century, but I was having a hard time finding sources, so could not confirm authenticity. I did find news reports about “Gemini” a Janus cat born in Oregon in June 2005. A female, she was born with two mouths, two tongues, two noses and four eyes. 
Janus cats are a rare occurrence. These cats live, on average, about four days. Despite my searches seeking to see how often diprosopus happens in cats, it was difficult locating the statistics I was hoping to find. That in itself seems to signify this condition's rareness in cats.