Duplications of the urinary collecting system and pyelo-ureteral junction obstruction (PUJO) are common, but the simultaneous presence of both anomalies is rarely encountered. In incomplete duplicated systems, PUJO usually affects the lower moiety. We present the case of a 5-year old boy with left bifid renal pelvis and hydronephosis of the lower moiety treated by robot-assisted pyeloureterostomy.