We report a rare case of an unruptured aneurysm arising from the junction between the internal carotid artery(ICA)and duplicate origin of the middle cerebral artery(MCA). A 49-year-old man who presented with vertigo was admitted to our hospital. Subsequent MR and CT angiograms revealed an incidental aneurysm arising from the proximal end of the duplicate origin of the left MCA. He underwent clipping via the distal transsylvian approach. The aneurysm projected to the posterior direction and several perforators from the two M1 segments were attached to the neck of the aneurysm, which made neck dissection very difficult. Approaching the space from the posterior/lateral direction to reach the M1 segment, the aneurysm neck was successfully dissected and clipped without sacrificing any perforators or stenosis of parent vessels. The postoperative course was uneventful, and the patient was discharged within 2 weeks. Duplicate origin of the MCA has been confused with a fenestration of the proximal M1 segment or terminal segment of the ICA. In this condition, two MCA branches arise separately from the terminal segment of the ICA and fuse early to form an arterial ring at the proximal M1 segment. An aneurysm originating from this anomaly has never been described. However, some aneurysms associated with this type of origin may have incorrectly been reported as those associated with proximal MCA or terminal ICA fenestrations. We summarize the cases that are similar to our case of duplicate origin of the MCA with an aneurysm, and discuss the precise diagnosis and significant intraoperative problems that were described in the literature.