Colonic duplication is a very uncommon congenital malformation. Most cases are detected during childhood and may occur with other systemic malformations. Since 1950, less than 90 cases of colonic duplication have been reported. In this study, we report the case of a 33-year-old female who was referred to our coloproctology service for evaluation. Since 16 years ago, she has noticed an intermittent left side painless abdominal mass that sometimes spontaneously disappears. In the last months, she reported that the frequency of this complaint has been more frequent. A barium enema X ray study revealed a significant dilation of the descendent and sigmoid colon. An exploratory laparotomy was performed and sigmoid colon duplication was found. The abnormal segment was removed and an end-to-end anastomosis was performed. There were no complications in the postoperative period and she was discharged at day 7. This case describes the difficulties in detecting this rare malformation that should be considered as part of the differential diagnosis of abdominal masses in adult patients.