  Tidal disruption events (TDEs) occur when a star enters the tidal radius of a
supermassive black hole (SMBH). If the star only grazes the tidal radius, a
fraction of the stellar mass will be accreted in a partial TDE (pTDE). The
remainder can continue orbiting and may re-disrupted at pericenter, causing a
repeating pTDE. pTDEs may be as or more common than full TDEs (fTDEs), yet few
are known. In this work, we present the discovery of the first repeating pTDE
from a systematically-selected sample, AT\,2020vdq. AT\,2020vdq was originally
identified as an optically- and radio-flaring TDE. Around $3$ years after its
discovery, it rebrightened dramatically and rapidly in the optical. The optical
flare was remarkably fast and luminous compared to previous TDEs. It was
accompanied by extremely broad (${\sim}0.1c$) optical/UV spectral features and
faint X-ray emission ($L_X \sim 3\times10^{41}$\,erg\,s$^{-1}$), but no new
radio-emitting component. Based on the transient optical/UV spectral features
and the broadband light curve, we show that AT\,2020vdq is a repeating pTDE. We
then use it to constrain TDE models; in particular, we favor a star originally
in a very tight binary system that is tidally broken apart by the Hills
mechanism. We also constrain the repeating pTDE rate to be $10^{-6}$ to
$10^{-5}$ yr$^{-1}$ galaxy$^{-1}$, with uncertainties dominated by the unknown
distribution of pTDE repeat timescales. In the Hills framework, this means the
binary fraction in the galactic nucleus is of the order few percent.
