Duplications of the small bowel are an uncommon group of congenital malformations whose symptoms rarely occur in adults. We report the case of a 21 year-old man who suffered during four years from symptoms related to the ulceration of ectopic gastric mucosa in an ileal duplication (spontaneously resolving peritonitis, pelvic abscess, recurrent gastrointestinal bleeding and chronic abdominal pain). Radiological and endoscopic explorations and a first laparotomy failed to provide the diagnosis. 99m pertechnetate (99mTcO4) scintigraphy demonstrated ectopic gastric mucosa and guided the radiological detection of an ileal ulceration. Injection of 99mTcO4 during a second laparotomy located the ectopic gastric mucosa at the level of a tumefaction found on the mesenteric side of the terminal ileum. Duplication was only recognized at pathological examination of the ileal resection. This case emphasizes the need to evoke a duplication in clinical circumstances like those described in this patient as well as the usefulness of 99mTcO4 in the diagnosis of such a malformation.