  Macrolensing of gamma-ray bursts (GRBs) is expected to manifest as a GRB
recurring with the same light curve and spectrum as a previous one, but with a
different flux and a slightly offset position. Identifying such lensed GRBs may
give important information about the lenses, cosmology, as well as the GRBs
themselves. Here we present a search for lensed GRBs among $\sim 2700$ GRBs
observed by the Fermi Gamma-ray Burst Monitor (GBM) during 11 years of
operations. To identify lensed GRBs, we perform initial cuts on position,
time-averaged spectral properties and relative duration. We then use the
cross-correlation function to assess the similarity of light curves, and
finally analyze the time-resolved spectra of the most promising candidates. We
find no convincing lens candidates. The most similar pairs are single-pulsed
GRBs with relatively few time bins for the spectral analysis. This is best
explained by similarities within the GRB population rather than lensing.
However, the null result does not rule out the presence of macrolensed GRBs in
the sample. In particular, we find that observational uncertainties and Poisson
fluctuations can lead to significant differences within a pair of lensed GRBs.

High Energy Astrophysical Phenomena