  The study of high energy cosmic rays requires detailed Monte Carlo
simulations of both, extensive air showers and the detectors involved in their
detection. In particular, the energy calibration of several experiments is
obtained from simulations. Also, in composition studies simulations play a
fundamental role because the primary mass is determined by comparing
experimental with simulated data. At the highest energies the detailed
simulation of air showers is very costly in processing time and disk space due
to the large number of secondary particles generated in interactions with the
atmosphere. Therefore, in order to increase the statistics, it is quite common
to recycle single showers many times to simulate the detector response. As a
result, the events of the Monte Carlo samples generated in this way are not
fully independent. In this work we study the artificial effects introduced by
the multiple use of single air showers for the detector simulations. In
particular, we study in detail the effects introduced by the repetitions in the
kernel density estimators which are frequently used in composition studies.
