  The discovery of a repeating Fast Radio Burst (FRB) source, FRB 121102,
eliminated models involving cataclysmic events for this source. No other
repeating FRB has yet been detected in spite of many recent FRB discoveries and
follow-ups, suggesting repeaters may be rare in the FRB population. Here we
report the detection of six repeat bursts from FRB 180814.J0422+73, one of the
13 FRBs detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)
FRB project during its pre-commissioning phase in July and August 2018. These
repeat bursts are consistent with originating from a single position on the
sky, with the same dispersion measure (DM), ~189 pc cm-3. This DM is
approximately twice the expected Milky Way column density, and implies an upper
limit on the source redshift of 0.1, at least a factor of ~2 closer than FRB
121102. In some of the repeat bursts, we observe sub-pulse frequency structure,
drifting, and spectral variation reminiscent of that seen in FRB 121102,
suggesting similar emission mechanisms and/or propagation effects. This second
repeater, found among the first few CHIME/FRB discoveries, suggests that there
exists -- and that CHIME/FRB and other wide-field, sensitive radio telescopes
will find -- a substantial population of repeating FRBs.
