  We search for signatures of gravitational lensing in the gravitational-wave
signals from compact binary coalescences detected by Advanced LIGO and Advanced
Virgo during O3a, the first half of their third observing run. We study: 1) the
expected rate of lensing at current detector sensitivity and the implications
of a non-observation of strong lensing or a stochastic gravitational-wave
background on the merger-rate density at high redshift; 2) how the
interpretation of individual high-mass events would change if they were found
to be lensed; 3) the possibility of multiple images due to strong lensing by
galaxies or galaxy clusters; and 4) possible wave-optics effects due to
point-mass microlenses. Several pairs of signals in the multiple-image analysis
show similar parameters and, in this sense, are nominally consistent with the
strong lensing hypothesis. However, taking into account population priors,
selection effects, and the prior odds against lensing, these events do not
provide sufficient evidence for lensing. Overall, we find no compelling
evidence for lensing in the observed gravitational-wave signals from any of
these analyses.
