Search for strongly lensed counterpart images of binary black hole
  mergers in the first two LIGO observing runs
  Strong gravitational lensing can produce multiple images of the same
gravitational-wave signal, each arriving at different times and with different
magnification. Previous work has explored if lensed pairs exist among the known
high-significance events and found no evidence of this. However, the
possibility remains that weaker counterparts of these events are present in the
data, unrecovered by previous searches. We conduct a targeted search
specifically looking for sub-threshold lensed images of known binary black hole
(BBH) observations. We recover candidates matching three of the additional
events first reported by Venumadhav et al. (2019), but find no evidence for
additional BBH events. We also find no evidence that any of the Venumadhav et
al. observations are lensed counterparts. We demonstrate how this type of
counterpart search can constrain hypotheses about the overall source and lens
populations and we rule out at very high confidence the extreme hypothesis that
all heavy BBH detections are in fact lensed systems at high redshift with
intrinsic masses < 15Msun.
