Duplex Kidney & Small Kidney

Hi, my almost 6yr old Daughter has suffered from many UTI's over the years. At 2.5yrs she had an ultrasouns which was unremarkable, both kidneys appeared normal and measured approx 7.3cms. However, this was a very brief ultrasound as she was scared and crying a lot.
Fast forward to March this year, I told my Dr I was concerned because, although she hasn't had a UTI for over a year, I have been worried that she doesn't go to the toilet much and when she does, there's not a lot coming out, even though she drinks well. He decided to do another ultrasound.
Briefly the results were:
Left - Complete duplication of the collecting system, left distal ureter showing mild dilation of 3.3mm. Kidney measures 9.3cm (95th percentile).
Right kidney appeared normal but measures 7.6cm (50th percentile).
Bladder appears normal. No sign of hydronephrosis.

There is a maternal history of Kidney problems, and my 4yr old was born was a left pelvic kidney (that is of a good size and seems normal in every other way). My oldest daughter's kidneys are fine.

My main concern is with the kidney that has only grown 0.3cm in 3.5 years. The Radiologist said it suggests possible kidney damage from the infections and that it may not be working, and that she should see a specialist (I am taking her irrespective because I worry). I am worried that both kidneys may be no good.
Can anyone shed some light on this, and/or share your experiences with similar situations.
Thanks and regards,
Claudine.