What kind of problems can a double collecting kidney cause?

None: A duplicated collecting system where you have two ureters coming from a single kidney occur commonly enough to be considered normal. They are usually discovered during studies for other reasons and cause no medical problems.

Can be problem free: 1% of population have complete duplication of collecting sytem ; ureter. Many more have just a duplex collecting system without any problem. Problems with double collecting sydtem referred to me, after a UTI os an incidental finding are: bladder reflux, hydroneprosis from uretero-pelvic junction obstruction involving both kidney elements or just lower one, upper pole megaureter or ureterocele.