prevent overlaps between draggables div that have the same class

i have a schedule web , i have week table and some draggable div that have a class name is "Block" so when i drag and drop this block , when i drop a block over another block in the week table , these 2 block are overlapping how do i prevent this overlapping ? [this is the overlapping of my dragging any idea to prevent this cinflict? thank you `

                <tr class="table-info">

                    <%    for (int j = 0; j < 6; j++) {
                    %>
                    <td id="cell<%=i%><%=j%>" class="cell" style="width: 150px; height: 20px;"></td>
                    <%}  %>
                </tr>
                <%}%>
            </table>`][1]