More than realise it. People with extra kidneys are often oblivious to their excess baggage, and only discover it after an ultrasound scan or surgery for a more urgent medical problem. Laura Moon, an 18-year-old from Whinmoor near Leeds, heard of her bonus kidneys during an ultrasound to investigate recurrent stomach pains. Her case is particularly unusual, as each of her kidneys appears to be fully formed, at around 11cm long.

"It's extremely rare for additional kidneys to be complete. One in a million is probably about right," says Niaz Ahmad, a transplant surgeon at St James's University Hospital in Leeds. "I've seen thousands of kidneys and I've never seen this."

The condition is caused by a glitch in the first trimester, when the developing kidneys split in two. It is more common for these "duplex kidneys" to split only partially, or to grow a second ureter (the tube that drains urine into the bladder). Moon is having tests to check her four kidneys all work properly. If they do, she may be able to donate one or two.

Duplex kidneys occur in 1% of the population, the most common complication being infections caused by urine flowing back up to the ureter. But some patients have been delighted with their extra organs. Two Latvian brothers found to have four kidneys each said it explained why they were able to drink their friends under the table.

Kidneys are not the only organs that can appear more often than they should. Around one in 1,000 women in the UK is born with two wombs. In 2006, Hannah Kersey from Northam in Devon gave birth to triplets from one of them. The same year, Reuters reported that an Indian businessman had checked into a New Delhi hospital to have one of his two penises removed. The condition, called diphallia, affects one in 5.5 million men.