  Realistic lightcone mocks are important in the clustering analyses of large
galaxy surveys. For simulations where only the snapshots are available, it is
common to create approximate lightcones by joining together the snapshots in
spherical shells. We assess the two-point clustering measurements of central
galaxies in approximate lightcones built from the Millennium-XXL simulation,
which are constructed using different numbers of snapshots. The monopole and
quadrupole of the real-space correlation function is strongly boosted on small
scales below 1 Mpc/h, due to some galaxies being duplicated at the boundaries
between snapshots in the lightcone. When more snapshots are used, the total
number of duplicated galaxies is approximately constant, but they are pushed to
smaller separations. The effect of this in redshift space is small, as long as
the snapshots are cut into shells in real space. Randomly removing duplicated
galaxies is able to reduce the excess clustering signal. Including satellite
galaxies will reduce the impact of the duplicates, since many small-scale pairs
come from satellites in the same halo. Galaxies that are missing from the
lightcone at the boundaries can be added to the lightcone by having a small
overlap between each shell. This effect will impact analyses that use very
small-scale clustering measurements, and when using mocks to test the impact of
fibre collisions.
