A transmission includes: a first input shaft selectively receiving power from an engine; a second input shaft continuously receiving power from the engine; a first output shaft arranged in parallel with the first input shaft and the second input shaft; a differential drawing power from the first output shaft and a second output shaft; a first shift unit providing desired gear shifts; a second shift unit to carry out duplicate gear shifts; and a one-way clutch to transmit power from the engine to the differential. In particular, the duplicate gear shifts has a gear ratio substantially identical to a gear ratio of one of the gear shifts made by the first shift unit, and the one-way clutch is arranged in a power transmission line for transmitting the power from the engine to the differential through the second shift unit.