This story is particularly shocking as we're often told the bond between twins is "unbreakable". Sometimes this bond takes on an almost magical quality, with stories of twin telepathy.
"Twins have many more similar experiences than your average siblings," explains Dr Avidan Milevsky, a psychologist and author of Sibling Issues in Therapy. "The similarities between them often create a certain bond that is unrepeatable in other sibling dynamics."
That is why it's so intriguing and horrifying when one tries to murder the other.
The tale of Alexandria and Anastasia is the latest twin murder to make headlines. But there have been a number over the years.
In 1998, San Diego student Gina Han masterminded a plot against her identical twin sister Sunny. The twins, who were co-valedictorians of their high school class had become estranged in recent years (Gina was convinced that Sunny had some of her belongings and wouldn't return them).
Gina and two friends hatched a plan to kill Sunny, but it was botched when police burst into the apartment to rescue Sunny and her flatmate (they were found bound and gagged). Gina, who was branded the "evil twin" in the press was sentenced to 26 years to life. She sobbed uncontrollably in the dock as she was sentenced.
In 2014 Robert Cerqua from the UK, was found guilty of stabbing his identical twin brother Christopher to death on New Year's Eve. A fight broke out after the 31-year-olds began arguing about their past. After stabbing Christopher in the torso with a kitchen knife Robert left the house barefoot and joined his girlfriend at a nearby party until 2.20am when he was arrested. He passed out drunk in the police car.
(A brief overview of the history of twin killings suggests that non-identical or fraternal twin murders seem to occur less than those committed by identical twins, but this may be because these less sensationalised cases tend not to make the headlines).
Dr Milevsky attributes wanting to murder your twin to a host of factors.
First, there is the element of sibling rivalry. Research studies indicate that up to 45 per cent of adults have a rivalrous or distant relationship with a sibling.
"Classic sibling dynamics may be a small element within this disturbing phenomenon but at the core, actually killing a sibling is driven by the killer possessing severe psychopathology and psychosis," he says.
"Add to this severe psychopathology, the competition that is often found in twin relationships, and you may then find a case of twin killing."
You might assume that having this twin bond would add to their sense of grief. However, this isn't always the case:
When Robert Cerqua woke up drunk in the back of the police car after killing Christopher he said: "Is he dead? You are lying, he cannot be".
While being taken to his cell, he added: "Was it me? Was it me? I cannot remember. Honestly, is my brother dead?"
When he was told the charge against him the next day in his cell, he allegedly said: "I killed my f****** brother. I don't want to be here any more.
"Nine times out of 10 it would be all right. [That's] just my luck, he had to die on me."
Twin loss expert and author of When Grief Calls Forth the Healing Mary R. Morgan says that guilt often follows relief.
"Relief can also surface at the end of a complex relationship, especially when there is a conflicted 'personal identity versus twin bond' issue."
So could murdering a twin be seen as an act of liberation?
"Some twin pairs - not all - become disillusioned about their twinship because they struggle to be 'known,' not just 'noticed,'" twin expert and The Same but Different author Dr Joan A. Friedman explains.
Indeed, some people refer to twins as "two halves of a whole," a tag that can frustrate twins struggling to define their own identity.
"Since outsiders relate to them as a unit, they expectedly have conflicts with their twin in an attempt to define their individual selves. Rivalry in adult twins has everything to do with wanting to be on one's own and discovering one's sense of self without worrying how this will disrupt and upset one's twin," Friedman says.
She says that most twin pairs eventually find ways to define their identity, but some twins aren't so lucky.
Take the case of Welsh twins June and Jennifer Gibbons, dubbed the "Silent Twins" due to their secret twin language and selective mutism. After a crime spree of arson and theft, the then 18 year olds were sent to Broadmoor, the UK's highest security psychiatric hospital, where they both became convinced that in order to experience true freedom, one had to die.
Entries in June's diary give some idea of how their twin bond trapped them, rather than strengthened them.
"Nobody suffers the way I do ... This sister of mine, a dark shadow robbing me of sunlight, is my one and only torment," she wrote.
Jennifer agreed, writing in her own diary: "We have become fatal enemies in each other's eyes," but she questioned if she could ever live without her twin:
"Can I get rid of my own shadow? Without my shadow, would I gain life, be free or left to die?"
One day, they told their only friend journalist Marjorie Wallace (author of their biography), that one of them wouldn't make it out of the hospital alive.
Jennifer looked at Wallace and said, "I'm going to die. We've decided."
Eleven years on, just hours after the twins were released from Boardmoor (they were being moved to a facility closer to Wales), Jennifer, at the age of 29, died on her sister's shoulder from a sudden inflammation of the heart muscle.
The cause remains a mystery.
