  Retrieving the mass of a gravitational-wave (GW) source is a fundamental but
difficult problem because the mass is degenerate with redshift. In astronomy,
three types of redshift exist, namely cosmological, Doppler, and gravitational
redshift, but the latter two are normally too weak to affect the observation.
In this Letter, we show that the current astrophysical models allow binary
black holes (BBHs) to merge within $10$ Schwarzschild radii of a supermassive
black hole (SMBH). We find that in this case both the Doppler and gravitational
redshift are significant, and in the most extreme condition they could increase
the "apparent" black-hole mass and distance by a factor of $1.9-3.4$. We show
that such a factor is consistent with the distribution in the distance-mass
diagram of the ten BBHs detected so far by LIGO/Virgo. We also discuss the
difficulties of this redshift scenario caused by the low event rate predicted
by the current models, as well the potential solutions.
