  Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb
lattice. We investigate the topological properties of a silicene superstructure
generated by an external periodic potential. The superstructure is a quantum
spin-Hall (QSH) insulator if it is topologically connected to silicene. It is
remarkable that two inequivalent K and K' points in the silicene Brillouin zone
are identified in certain superstructures. In such a case two Dirac cones
coexist at the same Dirac point in the momentum space and they are hexagonally
warped by the Coulomb interaction. We carry out a numerical analysis by taking
an instance of the ($3\times 3$) superstructure on the ($4\times 4$) structure
of the Ag substrate. We show that it is a QSH insulator, that there exists no
topological phase transition by external electric field, and that the
hexagonally warping occurs in the band structure.
