What is the origin of these extra diffraction spots? One immediate answer is double diffraction, which often occurs in thick two-phase materials when a diffracted beam travelling through a crystal is rediffracted when it passes into a second crystal. Hence, it would be natural to expect that double diffraction induces several extra diffraction spots in MSLs. We, however, argue that the simple double diffraction process cannot account for the extra diffraction spots shown in Fig. . Double-diffraction spots occur around each of the primary reflections, including the direct beam spot, by keeping the symmetry of the constituent crystals. This general feature of double diffraction implies that as for twisted hBN layers, two types of primary reflections, each with 6-fold rotational symmetry, should be accompanied by a set of double diffraction spots with the same 6-fold rotational symmetry, as indeed observed in 10° twist angle double-multilayer of ~100-nm-thick hBN sample. Contrary to the above expectation from double diffraction, the extra diffraction spots given in Fig. does not show the 6-fold rotational symmetry except for the nearly commensurate sample at  = 21.6°. Furthermore, for very thin specimens in which the WPOA holds, the double diffraction does not contribute to the entire diffraction process, as will be discussed below.