  We report distinctly double-peakedH-alpha and H-beta emission lines in the
late-time, nebular-phase spectra (>~200 d) of the otherwise normal at early
phases (<~ 100 d) Type IIP supernova ASASSN-16at (SN 2016X). Such distinctly
double-peaked nebular Balmer lines have never been observed for a Type II SN.
The nebular-phase Balmer emission is driven by the radioactive Co56 decay, so
the observed line-profile bifurcation suggests a strong bipolarity in the Ni56
distribution or in the line-forming region of the inner ejecta. The strongly
bifurcated blue- and red-shifted peaks are separated by ~3x10^3 km/s and are
roughly symmetrically positioned with respect to the host-galaxy rest frame,
implying that the inner ejecta are composed of two almost detached blobs. The
red peak progressively weakens relative to the blue peak, and disappears in the
740 d spectrum. One possible reason for the line-ratio evolution is increasing
differential extinction from continuous formation of dust within the envelope,
which is also supported by the near-infrared flux excess that develops after
~100 d.
