  A surprising finding of recent studies is the large number of Active Galactic
Nuclei (AGN) associated with moderately massive black holes ($\rm
\log(M_\bullet/M_\odot)\sim 6-8$), in the first billion years after the Big
Bang ($z>5$). In this context, a relevant finding has been the large fraction
of candidate dual AGN, both at large separations (several kpc) and in close
pairs (less than a kpc), likely in the process of merging. Frequent black hole
merging may be a route for black hole growth in the early Universe; however,
previous findings are still tentative and indirect. We present JWST/NIRSpec-IFU
observations of a galaxy at $z=7.15$ in which we find evidence for a $\rm
\log(M_\bullet/M_\odot)\sim7.7$ accreting black hole, as traced by a broad
component of H$\beta$ emission, associated with the Broad Line Region (BLR)
around the black hole. This BLR is offset by 620 pc in projection from the
centroid of strong rest-frame optical emission, with a velocity offset of
$\sim$40 km/s. The latter region is also characterized by (narrow) nebular
emission features typical of AGN, hence also likely hosting another accreting
black hole, although obscured (type 2, narrow-line AGN). We exclude that the
offset BLR is associated with Supernovae or massive stars, and we interpret
these results as two black holes in the process of merging. This finding may be
relevant for estimates of the rate and properties of gravitational wave signals
from the early Universe that will be detected by future observatories like
LISA.
