There are festivals for twins all over the world, from the huge gathering in Twinsburg, Ohio, in the United States, to the less celebratory Twinless Twins Convention in Florida. But the mainland has more reason than most to mark the marvel of twins. To legally have two children under a one-child policy is a blessing and something many couples dream about.

Mojiang county has been extra blessed. The world average for conceiving identical twins is about three in 1,000 births; in Mojiang, that figure is four in 1,000. And in the village of Hexi, the 100 resident families count a staggering 10 sets of identical twins among them.

The county has more than 1,500 identical twins and triplets in a population of only 380,000. This may not qualify as a scientific marvel but it is enough of an anomaly for the people of Mojiang to believe there's something in the water.

'It is the water,' insists Huang Fengqi, a resident of Hexi and mother of 13-year-old identical twin girls. 'I wanted twins and I drank the water like everybody in Hexi. We've always had a lot of twins in the village but there have been more people coming to drink the twin water in the past eight years.'

Legend has it that a long time ago, a spirit was enchanted by a beautiful girl and impregnated her. But an evil wizard wanted to kill the girl, so she was forced to run away from her village. She gave birth to twins in Hexi, and the babies transformed into two wells, which the villagers use for their water.

Mojiang is known as Twin City, Gemini City and the Home of Twins. It is also called 'the place where the sun turns', as the Tropic of Cancer cuts through the centre of town. Mojiang Hani Autonomous County is the only place where Hani are in the majority, making up 60 per cent of the population.

County leader Li Hong suggests the answer to the twins' mystery might be in Mojiang's unique characteristics. 'In Mojiang, we see twins, twin animals and twin plants everywhere,' he says. 'Experts say this phenomenon is probably related to our geographic position, the climatic conditions and inherited genes, but the specific reasons remain a mystery.'

There has been a significant rise in the number of multiple births since the 1980s, especially in America and Europe. This can largely be attributed to older women using fertility treatments. But such procedures are not common in Yunnan, where most women become mothers in their 20s.

Identical twins are the result of a single egg splitting after conception, which is much rarer than incidences of non-identical, or fraternal, twins, the result of two eggs being fertilised.

Globally, there has been little research into the science behind identical twins, but there is some suggestion the occurrence is higher in populations that consume quantities of yams, a vegetable that is grown in Hani rice fields.

Whatever the reason, the local government has used the phenomenon to launch the tourist 'twindustry', combining International Twins' Day and the Hani Solar Festival in a two-day event that starts on May 1.

Yunnan certainly knows how to put on a party. 'Bizarre' is too mild a word for the antics witnessed at the seventh Twins' Day, which features multitudes of identical twins, triplets and quadruplets, a riot of faces smeared black with ash, outlandish music and dance performances, bare-handed fish catching, tugs-of-war in a giant pool of mud, a mass wedding, multiple-birth-inducing magic water, some serious drinking and a dinner table set for 4,000.

The unofficial start is a banquet on the eve of the festival. Blond twins from Beijing, Jing Hong and Jing Xiang, the winners of last year's Gemini Twins Talent Competition and the official spokestwins of this year's festival, arrive to a frenzy of media attention.

The food is the least important feature of the banquet. There are quadruplet boys from Chuxiong, Yunnan, stunning triplets who look like movie stars and 23 pairs of twins from eight other countries. Bands of Hani women in colourful traditional costume surround each table and sing like sirens, cajoling those present to down glasses of the potent local speciality, zigujiu, a purple rice-based alcoholic drink.

'Hani people love to sing and drink,' says Bai Yunfei, a local celebrity singer performing at the banquet. 'We'll drink you to the floor and then sing you a lullaby.' He downs another glass of the region's green olive wine with a table of female Russian identical twosomes that he has just serenaded with a Hani love song.

After dark, hundreds of twins and thousands more twin-spotters head to Taiyang Guangchang (Sun Square) for the opening of the Hani Solar Festival, which began in 2005 and was twinned with Twins' Day the following year.

'It was started so all the different Hani people could come together to have fun, and to promote Hani culture for tourism,' local official Jiangxi says.

On a stage covered with straw, troupes of men and women from all corners of the county entertain the crowds with folk songs and dances. The clothes are stunning; black, navy and blue cloth embroidered with intricate, colourful patterns. Shining silver discs, studs and old colonial French, Burmese and Indian coins cover jackets, all signifying prosperity. Foot-high conical headdresses are covered with silver balls, multicoloured beads, feathers, long red furry tassels, more silver coins and other bits of metal.

'Our jackets and hats are works of art,' says a village leader.

The songs and dances are all about the rituals of village life; picking and planting tea, and love; boy serenades girl, girl seduces boy. The music is played on drums, cymbals and Jew's harps. In front of the stage, young twins jump up to dance. They mimic the moves and give the pack of photographers a field day.

'My girls are having so much fun,' says Asif Alim, the father of Shazia and Nureem Alim, three-year-old identical twins from India. 'They get to meet so many twins and the culture here is so different and exciting.' Asif is a travel agent in Kolkata. He was contacted by a festival organiser looking for participants. 'I couldn't believe my luck. I was very happy to bring my girls to China.'

'They don't take the performance too seriously,' says Kateule Kate Nguluta, 23, a twin from Zambia. 'They just love the music and to dance. It's like Africa.'

Her fraternal twin, Chanda Margaret, adds, 'The crowd feels it so they join in the fun.'

What follows is a swirling sea of dancers around three big bonfires. Spiky-haired teenagers link arms with old women in brilliant red-feather headdresses. Striking Russian doppelgangers dance with suave, mirror-image Philippine twins in dazzling Hani waistcoats. Teenagers, young men, old women: all break off into their own smaller, more manageable dance circles around the square and spill into the surrounding streets.

The next morning, the Gemini Twins Talent Competition kicks off Twins' Day in a Mojiang television studio. The first round focuses on the children. Many of Yunnan's 24 minority groups are represented by double-acts kitted out in the full splendour of their culture's costumes.

Two little Dai boys in baby-blue silk, with shiny gold headbands, sing a folk song. What they lack in tone they make up for in volume and enthusiasm. Teenage Mojiang twin girls sing a Hani song so high-pitched that other children cover their ears. Two young Bai boys give a painful recital on gourd flutes, but to big applause.

Identical teenage Laotian girls sing in their native tongue and two blond Russian girls dressed as snow princesses do a high-kicking Sugar Plum Fairy piece.

Later, there's a mass wedding in the sprawling Tropic of Cancer Park. Thirty couples are inspired by the promise: 'Mysterious twin wells, wonderful twin beds and unique Hani food are the most likely for you to achieve your good wishes of having twins. Tropic of Cancer, witness the happy life.'

The couples exchange vows and are blessed by identical twin Hani children, who rub black ash on their faces, a local tradition that is said to bring good luck.

The couples then wend their way to Hexi and Twins' Park, home to the multiple-birth-inducing twin wells.

Two 10-metre fetus statues guard the entrance. The couples are also greeted by a sculpture of a split ovum revealing a pair of identical embryos. The newlyweds drink magic water and are given twin Pu'er tea and more twin-inducing paraphernalia.

The Twins' Parade precedes Mojiang's plunge into pandemonium. Hani men playing the Jew's harp and village elders in ceremonial finery announce the parade's arrival. Jing twins Hong and Xiang follow in a big red hot rod. Next is a caravan of huge floats filled with happy waving twosomes. The last float carries the newlyweds, who are followed by 100 giant drums and 200 bare-chested Hani drummers. Then a horde of local boys in loin cloths smeared with bright war paint shake tall staffs covered in leaves and make a sound like a plague of cicadas.

There is no official signal for all hell to break loose; it just does. Thousands of people are suddenly armed with state-supplied bags of black ash. Sun Square turns into a scene from the movie Braveheart. Bands of warrior Hani do battle armed with fistfuls of ash. The 'people mountain, people sea' battle rages, engulfing the entire town. Everyone except the elderly is fair game. Government officials and police are the most ferocious. County boss Li leads a squadron of well-disciplined local officials in tit-for-tat guerilla blackenings against a company of Russian and South Korean twins.

'Foreigners will be very popular during this activity,' warns local official Jiangxi, meaning they will be targets for mobs of youths hell-bent on coating every inch of foreign skin a crusty black.

Black is an important colour for the Hani. Girls with blackened faces grow prettier, old men live longer and healthier. Blacken the face of a friend and it signifies a lasting friendship.

Inspired by the traditional Hani New Year street banquet, the longest boulevard of Mojiang's slick new shopping district, Bu Xing Jie (Walking Street), is set up for the evening meal. The table for 4,000 snakes through the centre of the city, set with an endless spread of local delicacies. Twins, their families, the newly-weds and all the Hani performers are invited to the feast. Troupes of Hani singers and dancers work their way along the tables, encouraging another night of double-vision revelry on the purple rice alcohol.

'This is amazing,' say beautiful 23-year-old Turkish twins Filiz and Deniz Unal. 'There are so many international people. We're surprised to see this number of twins. We'd love to come back next year.'

NEXT MORNING, THE HANI GO back to their roots for a day of festivities in Loulong, a short bus ride from Mojiang. It is a picturesque village, with traditional houses split between two hillsides facing a long narrow valley. Twins, villagers, tourists, dignitaries, officials and the media arrive at the village fairground; an Olympic swimming pool of thick wet mud, surrounded by a field of more mud.

City women in leather miniskirts and high heels teeter about in the muck. A young boy, naked from the waist down, throws a dead fish into the air and catches it. Everyone who's anyone is in Loulong to see the tug of war and bare-handed fishing contest. The former is won by the strong men and women of the local education department, while the latter turns into mass mud-splashing mayhem.

The final of the Gemini Twins Talent Competition is held that evening in Mojiang's sports stadium. This is serious business - destined to be the American Idol for twins the world over. Only the most dedicated twins and multiples make it to this stage. The arena is packed. Everyone has been issued with a lightsaber and plastic hand clappers are out in force. CCTV cameras catch the spectacle as they swivel on giant mechanical arms over the crowds.

Yi minority teenage triplets from Yuxi city do an 1980s-style techno dance in spandex. Twin Lijiang women, obviously professionals, sing a traditional number in Ta Liu minority outfits. Two young twins from Sichuan, in spangled pink dresses, do an elegant and well-choreographed ballet piece. Ten finalists perform before a row of deadpan judges, complete with harsh Simon Cowell-style criticism.

The Russian twins take it up a notch. They prowl onto the stage in tight, backless, leopard-print catsuits, suggestive tails swishing. Not surprisingly, they have the full attention of every adult male in the crowd as they grind their hips and thrust their tails into the air. Did nobody tell them this was a family affair?

The Russian girls are declared the winners. They'll be the spokestwins for the eighth International Twins' Festival in Twins City; that should be well worth a look.