Duplex collecting system is a common congenital anomaly of the urinary tract. This case shows the relationship of both moieties ureters according to Weigert-Meyer law, which states that:

• upper moiety ureter has an ectopic insertion medial and inferior to the lower moiety ureter and ends in a ureterocele
• lower moiety ureter has insertion lateral and superior to the upper moiety ureter - see the first axial delayed CT for relationship

See illustration here (rID:30185).