An interesting extension to the conventional Doppler shift experiment is described. The basic equipment is similar to other X-band systems and is built around a 4-port directional coupler. By using a vibrating mirror in place of the normal constant velocity reflector, the output from the mixer can be observed to occur at both the expected Doppler frequency and at double this frequency, depending on the position of the mirror. This rather surprising result is discussed in terms of a pseudo-standing-wave model and also in terms of mixing the Doppler shifted frequency with the transmitted frequency.