Gastrointestinal duplications are uncommon developmental abnormalities that occur anywhere along the gastrointestinal tract. We report a 36 year-old female admitted to the emergency room due to abdominal pain. Computed abdominal tomography demonstrated small bowel loops surrounding a tubular cystic structure and peritoneal free fluid. The surgical exploration revealed multiple malignant implants covering the visceral and parietal peritoneum and infiltrating completely the omentum. At the ileal mesentery we found a tubular cystic whitish tumor measuring 12 cm of diameter and 15 cm on length. A complete resection of the tumor was not considered an option due to the extensive peritoneal dissemination. Thirty-four days after the operation the patient died. The histopathology of the cystic wall was compatible with the architecture of intestinal wall extensively infiltrated by a moderately differentiated mucinous adenocarcinoma; a mucosal lining in parts atrophic and in parts infiltrated or replaced by adenocarcinoma was observed. A well structured muscular layer was recognized, and the myenteric plexus was identified.