  Gravitational wave (GW) echoes, if they exist, would be a probe to the
near-horizon quantum structure of black hole (BH), which has motivated the
searching for the echo signals in GW data. We point out that the echo
phenomenology related with the potential structure might be not so simple as
expected. In particular, if the near-horizon regime of BH is modelled as a
multiple-barriers filter, the late-time GW ringdown waveform will exhibit the
mixing of echoes, even the superpositions. As a result, the amplitudes of
successive echoes might not drop sequentially.
