  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.

High Energy Astrophysical Phenomena