Ten young people pose for a group shot, boys in front, girls behind. Almost identically dressed, they stare straight to camera with a mixture of shyness and defiance familiar from school photographs the world over. But the brightness of their uniform, the playfulness of their headgear, suggests that this is no ordinary gathering – and indeed it is not. These children, togged out in their holiday best, were among more than 2,000 sets of twins who poured into the Nigerian town of Igbo-Ora last autumn for the state of Oyo’s first twins festival – an event celebrating the town’s claim to be the twins capital of the world.

A group of twins pose during the maiden edition of Twins festival in Igbo-Ora, where more than 2,000 sets of twins gathered.

Twins play with a mirror at an orphanage in Gwagwalada. Right; sisters at the Igbo-Ora high school.

Just as the festivities were gearing up, two photographers happened to arrive in Oyo on their own twins mission. Bénédicte Kurzen and Sanne De Wilde were travelling the length and breadth of Nigeria to investigate the mythology that has grown up around a strange genetic anomaly: for it is not only Igbo-Ora that is blessed with unusual numbers of multiple births. “West Africa and specifically Yorubaland (the south west of Nigeria) has 10 times more twins than any other region in the world,” they say. Alongside portraits of beautiful young twins revelling in their doubleness, Kurzen and De Wilde’s haunting photo essay, Land of Ibeji, also reveals a more troubling history in certain parts of Nigeria, captured in a picture of two little girls lying together on a tiled floor. One is listlessly awake and the other sleeps, their bodies strewn with the brilliant red petals of a flamboyant tree as if they might be on the point of death, or involved in some sort of sacrificial rite.

Sisters at an orphanage in Gwagwalada founded by Olusola Stevens and his wife as a shelter for children at risk of infanticide because of negative attitudes to multiple births.

This disturbing photograph is one of several taken in an orphanage near the capital, Abuja, which has become a sanctuary for children in danger of being killed because of deeply held superstitions in their communities. Not for nothing did Chinua Achebe write, in his great 1958 novel Things Fall Apart, of twins being stuffed into earthenware pots and cast away into the forest because “the Earth has decreed that they were an offence on the land and must be destroyed”.

Brothers next to their home in the old city of Igbo-Ora.

The project takes its title from the Yoruba word for twin, though ibeji has come to have a significance far beyond an accident of birth. The explanatory text says that it means “double birth” and “the inseparable two”. It has its own orisha – or deity – in the Yoruba pantheon, and stands for the ultimate harmony between two people.

Sisters dressed in white for a ceremony at the Celestial church on a rainy season afternoon.

The photographers’ mission itself goes beyond the recording of a curious phenomenon. Through their pictures, they say, they hope “to open the eyes to the twin as a mythological figure and a powerful metaphor: for the duality within a human being and the duality we experience in the world that surrounds us”. In this way, “the mythology of twinhood becomes a way to address themes like identity, genetics, demographics, economy, religion and environmental issues”.

Sisters pose for the photographers next to their home.

Kurzen, a French documentary photographer who has been based in Nigeria for the past seven years, explains that she had for a long time wanted to investigate its twins phenomenon, but had been unsure how to do so until De Wilde, a Belgian now based in the Netherlands, joined her photo agency, Noor. De Wilde’s photographic specialism is genetics (her past work includes the stunning book The Island of the Color Blind, exploring life through the eyes of a Pacific island people who are all genetically colour blind).

Together they identified three places in Nigeria where multiple births had particular resonance: Igbo-Ora, where twins are celebrated; Gwagwalada, whose orphanage stands as terrible testimony to continuing infanticide; and Calabar, a southern city where a 19th-century Scottish Presbyterian missionary, Mary Slessor, is honoured for persuading local people to cherish, rather than kill, their twins.

Two young students pose next to their house in the old city of Igbo-Ora.

“In Europe,” Kurzen says, “we have replaced our ancient mythologies. Nobody really knows what Castor and Pollux or the Gemini sign mean any more. We come to Nigeria to revive that mythology.” As the project progressed, they found themselves being mistaken for twins and working like them: “There was no question of ego at all. One of us would have an idea and the other would complete it. One would have the camera and the other would hold the light.”

A child plays with the laundry drying in the sun in Gwagwalada.

Twins pose in the old city of Igbo-Ora.

When I point out that some of their portraits resemble the work of Kehinde Wiley, the American portraitist of Barack Obama, Kurzen laughs and says they met him briefly in Lagos. Wiley is himself a twin.

In Land of Ibeji, they explain that Kehinde is the name given to second-born twins, but that in a witty twist of Yoruba mythology they are considered the older ones, who push their younger sibling out first to check out the world. The explanation is accompanied by a photograph of two little eyes peeping through holes in a white cloth fringed with royal blue. A masquerader’s mask, perhaps, or two about-to-be-born twins checking out the world.

Land of Ibeji by Bénédicte Kurzen and Sanne De Wilde is at TJ Boulting Gallery, London W1 until 29 June. A book of the series will be published in September by Trolley Books.