Upper pole nephrectomy has been the traditional surgical management of children with poorly functioning upper pole moieties in duplex renal collecting systems having ureteral ectopia and ureterocele. However, ablative surgery confers a risk of functional loss to the remnant moiety due to vasospasm or vascular injury. The authors of this paper hypothesise that ipsilateral ureteroureterostomy (IUU) is a safe and feasible approach for the management of these patients and that residual function in the obstructed upper pole does not affect surgical outcomes and reviewed all patients with duplex systems who underwent IUU between 2010 and 2016 retrospectively. Patients were sorted into two groups based on preoperative imaging: those having <10% upper pole moiety function (UPMF) and those having >10% UPMF. Of the 53 children with ectopia or ureterocele affecting the upper pole in a duplex system, 21 had UPMF <10% (median function 0% and median age 1.49 years) and 32 had UPMF >10% (median function 15% and median age 0.91 years). Median follow-up was 27.4 months and 27.6 months. Statistical analysis revealed no significant differences in outcomes and no patient required secondary surgery thereby confirming the hypothesis. This paper demonstrates that this procedure is a safe and viable alternative to traditional heminephrectomy and can be considered even in cases with poor moiety function.