Duplex kidney is a developmental anomaly characterised by incomplete fusion of upper and lower pole moieties resulting in complete or incomplete duplication of the collecting system. At one extreme there is just duplication of the renal pelvis, draining via a single ureter. At the other end of the spectrum, two separate collecting systems drain independently.

Weigert-Meyer law stipulates that the upper pole moiety inserts ectopically, lower and medial to the lower pole moiety which inserts orthotopically. In boys the upper moiety inserts typically in the prostatic urethra (click here), and in girls either in to the urethra or directly into the vagina.
The ectopic insertion often has a urethrocoele which obstructs its own collecting system, and can distort the orthotopic lower pole moiety insertion such that it is prone to reflux.

1. eMedicine
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Credit: Dr Frank Gaillard