The duplex kidney is one of the common congenital anomalies of the kidney and urinary tract. Meanwhile, renal cell carcinoma is also the most typical variant constituting more than 90% of cancers of the kidneys. However, renal cell carcinoma is rare in a duplicated collecting system, and literature regarding its association is very sparse. Herein, we present a 28-year-old male patient who presented with abdominal pain and hematuria and was diagnosed with left upper pole renal cell carcinoma coexisted with a duplicated collecting system by computed tomography scan. The patient underwent a left partial nephrectomy followed by meticulous suturing of opened pelvicalyceal system. The histopathology result showed clear cell carcinoma. The postoperative period was uneventful, and the patient was discharged without complications. In conclusion, the coexistence of the duplex system and renal cell carcinoma in the ipsilateral kidney is rare and maybe more than a coincidence, requiring a deeper insight and further elucidation.