  The fractional Josephson effect has been observed in many instances as a
signature of a topological superconducting state containing zero-energy
Majorana modes. We present a nontopological scenario which can produce a
fractional Josephson effect generically in semiconductor-based Josephson
junctions, namely, a resonant impurity bound state weakly coupled to a highly
transparent channel. We show that the fractional ac Josephson effect can be
generated by the Landau-Zener processes which flip the electron occupancy of
the impurity bound state. The Josephson effect signature for Majorana modes
become distinct from this nontopological scenario only at low frequency. We
prove that a variant of the fractional ac Josephson effect, namely, the
low-frequency doubled Shapiro steps, can provide a more reliable signature of
the topological superconducting state.
