Dental anomalies can be classified in different groups: anomalies of volume, anomalies of number, anomalies of form, anomalies of position and anomalies by union. Of the latter, we distinguish between fusion, alveolus-dental gemination, concrescence, coalescence and anchylosis. Gemination is more frequent in the anterior teeth, although it can also affect the bicuspids and molars, being an anomaly of infrequent union (prevalence 0.5%). We present the case of a young male patient age 19, without medical antecedents of interest, that goes to consultation for repeated inflammatory accidents at level of the inferior left retromolar area. These episodes are caused by a semi-impacted inferior third molar that is fused to a supernumerary fourth molar, sharing its roots, crown, pulp chambers and canals. After the appropriate radiologic study and suitable planning, the semi-impacted third molar was extracted under local anaesthesia and without any other complications during or after the operation.