Angel's health deteriorated shortly after she was born
Five-year-old Angel Burton went for surgery after being beset by kidney problems since birth.
But surgeons at Sheffield Children's Hospital were astonished to find she had a pair of perfectly-formed organs sitting on top of the old ones.
Furthermore, the extra organs had taken over the work of the originals - so instead of suffering kidney failure, she could look forward to a bright, healthy future.
Angel's mum, Claire Burton, told the Daily Mail: "It's a real miracle. We're just so grateful to have Angel back to her happy, healthy self."
She said the surgeon told her he was "stunned" regular scanning over a five-year period had failed to pick up on the extra organs.
It is thought they were not detected as the new kidneys were sitting just above the originals and tests showed them as one.
Angel and her mother are looking forward to a healthy future
Angel, from Louth in Lincolnshire, suffers from duplex kidney, a rare condition where the organs are fused together in the middle. They either share or have their own ureters taking urine to the bladder.
In an even rarer occurrence, both of the organs were duplex, giving her four kidneys and four ureters.
Angel's health deteriorated almost as soon as she was born.
After being diagnosed with bilateral reflux - which means urine was leaking into and infecting her kidneys - a scan had revealed the organs had been badly damaged.
It was during an operation to create an artificial valve in October 2007 that doctors found the extra kidneys.
Angel is now eight and on course to make a full recovery.