This is a Guardian article and here is an extract from 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."
You can reduce your risk of kidney failure by avoiding taking prescription drugs that cause sodium retention, e.g. steroids like prednisolone, and antidepressants like amitriptyline, unless they are absolutely necessary, and by reducing your salt/sodium intake to a minimum.