def _collapse_duplicate_points(self):
<EOL>j_sorted = np.lexsort(keys=(self.x, self.y))<EOL>mask_unique = np.hstack([<EOL>True,<EOL>(np.diff(self.x[j_sorted]) != <NUM_LIT:0>) | (np.diff(self.y[j_sorted]) != <NUM_LIT:0>),<EOL>])<EOL>return j_sorted[mask_unique]<EOL>
Generate index array that picks out unique x,y points.

        This appears to be required by the underlying delaunay triangulation
        code.