  We present extensive, well-sampled optical and ultraviolet photometry and
optical spectra of the Type Ia supernova (SN Ia) 2017hpa. The light curves
indicate that SN 2017hpa is a normal SN Ia with an absolute peak magnitude of
$M_{\rm max}^{B} \approx$ -19.12$\pm$0.11 mag and a post-peak decline rate \mb\
= 1.02$\pm$0.07 mag. According to the quasibolometric light curve, we derive a
peak luminosity of 1.25$\times$10$^{43}$ erg s$^{-1}$ and a $^{56}$Ni mass of
0.63$\pm$0.02 $M_{\odot}$. The spectral evolution of SN 2017hpa is similar to
that of normal SNe Ia, while it exhibits unusually rapid velocity evolution
resembling that of SN 1991bg-like SNe Ia or the high-velocity subclass of SNe
Ia, with a post-peak velocity gradient of $\sim$ 130$\pm$7 km s$^{-1}$
d$^{-1}$. Moreover, its early spectra ($t < -7.9$ d) show prominent
\CII~$\lambda$6580 absorption feature, which disappeared in near-maximum-light
spectra but reemerged at phases from $t \sim +8.7$ d to $t \sim +11.7$ d after
maximum light. This implies that some unburned carbon may mix deep into the
inner layer, and is supported by the low \CII~$\lambda$6580 to
\SiII~$\lambda$6355 velocity ratio ($\sim 0.81$) observed in SN 2017hpa. The
\OI~$\lambda$7774 line shows a velocity distribution like that of carbon. The
prominent carbon feature, low velocity seen in carbon and oxygen, and large
velocity gradient make SN 2017hpa stand out from other normal SNe Ia, and are
more consistent with predictions from a violent merger of two white dwarfs.
Detailed modelling is still needed to reveal the nature of SN 2017hpa.
