code: def unique_sites(self, scaled_positions, symprec=1e-3, output_mask=False):
          
          scaled = np.array(scaled_positions, ndmin=2)
          symnorm = self.symmetry_normalised_sites(scaled)
          perm = np.lexsort(symnorm.T)
          iperm = perm.argsort()
          xmask = np.abs(np.diff(symnorm[perm], axis=0)).max(axis=1) > symprec
          mask = np.concatenate(([True], xmask))
          imask = mask[iperm]
          if output_mask:
              return scaled[imask], imask
          else:
              return scaled[imask]
docs: Returns a subset of *scaled_positions* containing only the
          symmetry-unique positions.  If *output_mask* is True, a boolean
          array masking the subset is also returned.
  
          Example:
  
          >>> from ase.lattice.spacegroup import Spacegroup
          >>> sg = Spacegroup(225)  # fcc
          >>> sg.unique_sites([[0.0, 0.0, 0.0], 
          ...                  [0.5, 0.5, 0.0], 
          ...                  [1.0, 0.0, 0.0], 
          ...                  [0.5, 0.0, 0.0]])
          array([[ 0. ,  0. ,  0. ],
                 [ 0.5,  0. ,  0. ]])