  We discuss strong gravitational lensing of gravitational waves from merging
of massive black hole binaries in the context of the LISA mission. Detection of
multiple events would provide invaluable information on competing theories of
gravity, evolution and formation of structures and, with complementary
observations, constraints on H_0 and other cosmological parameters. Most of the
optical depth for lensing is provided by intervening massive galactic halos,
for which wave optics effects are negligible. Probabilities to observe multiple
events are sizable for a broad range of formation histories. For the most
optimistic models, up to 4 multiple events with a signal to noise ratio >= 8
are expected in a 5-year mission. Chances are significant even for conservative
models with either light (<= 60%) or heavy (<= 40%) seeds. Due to lensing
amplification, some intrinsically too faint signals are brought over threshold
(<= 2 per year).
