Transmission for vehicle

A transmission includes: first and second input shafts receiving power from an engine; an output shaft arranged in parallel with the first input shaft; first and second countershafts arranged in parallel with the first and second input shafts, respectively; a first shift unit making desired gear shifts between the first input shaft and the output shaft, and between the first input shaft and the first countershaft; a second shift unit arranged between the second input shaft and the second countershaft in order to perform of duplicate gear shifts; a power connection unit transmitting power from the first countershaft to the output shaft; and a one-way clutch transmitting power from the engine to the power connection unit through the second shift unit. In particular, each duplicate gear shift has a gear ratio substantially identical to at least one gear ratio of the gear shifts of the first shift unit.