  The fast radio burst (FRB) population is observationally divided into sources
that have been observed to repeat and those that have not. There is tentative
evidence that the bursts from repeating sources have different properties than
the non-repeating ones. In order to determine the occurrence rate of repeating
sources and characterize the nature of repeat emission, we have been conducting
sensitive searches for repetitions from bursts detected with the Australian
Square Kilometre Array Pathfinder (ASKAP) with the 64-m Parkes radio telescope,
using the recently commissioned Ultra-wideband Low (UWL) receiver system, over
a band spanning 0.7$-$4.0 GHz. We report the detection of a repeat burst from
the source of FRB 20190711A. The detected burst is 1 ms wide and has a
bandwidth of just 65 MHz. We find no evidence of any emission in the remaining
part of the 3.3 GHz UWL band. While the emission bandwidths of the ASKAP and
UWL bursts show $\nu^{-4}$ scaling consistent with a propagation effect, the
spectral occupancy is inconsistent with diffractive scintillation. This
detection rules out models predicting broad-band emission from the FRB
20190711A source and puts stringent constraints on the emission mechanism. The
low spectral occupancy highlights the importance of sub-banded search methods
in detecting FRBs.
