It is one of the most common congenital renal tract abnormalities, characterized by incomplete fusion of the upper and lower pole moieties resulting in complete or incomplete duplication of the collecting system.

Duplication can be variable: At one end of the spectrum, there is merely duplication of the renal pelvis, draining via a single ureter. At the other extreme, two separate collecting systems drain independently into the bladder or ectopically. Duplex systems may be unilateral or bilateral and can be associated with a variety of other congenital abnormalities of the urinary tract, e.g., ureterocele.

Most duplicated systems are asymptomatic and diagnosed incidentally. However, where symptoms do occur (infection, reflux, or obstruction), the patient is likely to have completely duplicated ureters. Occasionally, hydronephrosis can be severe enough to result in flank discomfort or even a palpable mass.