A 21-year-old female patient was referred to the Department of Operative Dentistry of the University of Rostock because of complaints in the right maxilla indicating pulpitis in a maxillary molar. Tooth 17 had an extra cusp with an additional root on the buccal aspect. The point of fusion of the additional root with the regular tooth created an environment conducive to caries. The pulp chamber of the supernumerary root was opened during caries excavation and the clinical examination suggested a separate, supernumerary root canal. A standard radiograph showed no extraordinary root canal configuration, so during the first appointment only the supernumerary root canal was treated. Owing to continuing discomfort, a more precise radiological diagnosis was made using computed tomography (CT). The CT scan clearly showed that the separate root canal under the extra cusp communicated with the regular root canal system. Therefore, the diagnosis 'double teeth with incomplete gemination' was made and followed by root canal treatment for all the root canals. Keywords: dental anatomy, fusion, gemination, hyperdontia, supernumerary teeth