An incomplete-type ureteral duplication (IUD) is a congenital anomaly, and is usually found incidentally on imaging findings in children. Clinically, a marked megaureter is caused by a long-term 'yo-yo' phenomenon in IUD, and is a rare, initial suspicion in elderly people, especially when manifested through repeated urinary tract infections (UTI). We report on a 67-year-old woman who suffered from repeated UTI and left flank pain. IUD with marked megaureters resulting from the 'yo-yo' phenomenon was diagnosed by a series of imaging studies. The symptoms were successfully relieved by tapering and reimplantation of the dilated ureters. Finally, we emphasize a paradoxical phenomenon in the interpretation of the effective renal plasma flow study, even if this means correcting for the pitfalls of the examination itself.