Each kidney usually has one ureter connecting it to the bladder. According to The Children's Hospital of Philadelphia, people with a duplex kidney have an extra ureter that either empties directly into the bladder or connects to the other ureter. If the two ureters are connected, they form a single ureter that drains into the bladder.
In most cases, duplex kidney does not cause any medical problems. Some people with this variant may experience vesicoureteral reflux, which is when urine backs up from the bladder to the kidneys. Duplex kidney may also be associated with ureterocele, a swelling at the bottom of the ureter. If the swollen area gets too large, it can block the flow of urine from the kidneys to the bladder.