  The Majorana zero-energy modes (MZMs) residing at the boundary of topological
superconductors have attracted a great deal of interest recently, as they
provide a platform to explore fundamental physics such as non-Abelian
statistics, as well as fault-tolerant quantum computation. Period doubling of
Shapiro steps in a Josephson junction under microwave irradiation has been
regarded as strong evidence for the emergence of the MZMs at the junction
edges. However, questions remain as to how the Shapiro steps respond to the
presence of a 4{\pi}-periodic Josephson current. In this study, we investigated
the characteristic features of Shapiro steps with respect to the ratio
({\alpha}) of the 4{\pi}-periodic current to the topologically trivial
2{\pi}-periodic one, as well as the reduced microwave frequency ({\Omega}) and
McCumber parameter ({\beta}) of the junction. Our analysis reproduced Shapiro
steps similar to those observed experimentally for specific parameter sets of
{\alpha},{\Omega} ({\lesssim 0.1}), and {\beta} ({gtrsim 1.0}). Full
suppression of the first lobe of the n=1 step guarantees the presence of a
4{\pi}-periodic Josephson current.In addition, we discuss the range of {\Omega}
and {\beta} needed for full suppression of the first lobe of the {n=1} step,
even for small {\alpha} ({<0.1}). To observe "period-doubled Shapiro steps",
even with a small {\alpha}, the junction should have a large {I_c}{R_N} product
and sufficiently large junction capacitance.
