  Two of the dominant channels to produce merging stellar-mass black-hole
binaries are believed to be the isolated evolution of binary stars in the field
and dynamical formation in star clusters. The first reported black-hole binary
event from the third LIGO/Virgo observing run (GW190412) is unusual in that it
has unequal masses, nonzero effective spin, and nonzero primary spin at 90\%
confidence interval. We show that this event should be exceedingly rare in the
context of both the field and cluster formation scenarios. Interpreting
GW190412 as a remnant of a previous black-hole merger provides a promising
route to explain its features. If GW190412 indeed formed hierarchically, we
show that the region of the parameter space that is best motivated from an
astrophysical standpoint (low natal spins and light clusters) cannot
accommodate the observation. We analyze public GW190412 LIGO/Virgo data with a
Bayesian prior where the more massive black hole resulted from a previous
merger, and find that this interpretation is equally supported by the data. If
the heavier component of GW190412 is indeed a merger remnant, then its spin
magnitude is $\chi_1=0.56_{-0.21}^{+0.19}$, which is higher than the value
previously reported by the LIGO/Virgo collaboration.
