  Early optical observations of gamma-ray bursts can significantly contribute
to the study of the central engine and physical processes therein. However, of
the thousands observed so far, still only a few have data at optical
wavelengths in the first minutes after the onset of the prompt emission. Here
we report on GRB\,190106A, whose afterglow was observed in optical bands just
36 s after the {\em Swift}/BAT trigger, i.e., during the prompt emission phase.
The early optical afterglow exhibits a bimodal structure followed by a normal
decay, with a faster decay after $\sim \rm T_{0}+$1 day. We present optical
photometric and spectroscopic observations of GRB\,190106A. We derive the
redshift via metal absorption lines from Xinglong 2.16-m/BFOSC spectroscopic
observations. From the BFOSC spectrum, we measure $z= 1.861\pm0.002$. The
double-peak optical light curve is a significant feature predicted by the
reverse-forward external shock model. The shallow decay followed by a normal
decay in both the X-ray and optical light curves is well explained with the
standard forward-shock model with late-time energy injection. Therefore,
GRB\,190106A offers a case study for GRBs emission from both reverse and
forward shocks.
