problem after Update Panel triggering for dropdown to fill textboxes

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VB

ASP.NET

I am using update panel for dropdown to fill textbox. It works fine but the problem is after dropdown selected index change occurs my page design gets changed like the controls in the content template moves top to my page and there occurs duplication of controls in content template makes my design worse. how to resolve it

here is the code
thanks in advance

<tr align="left">
           <td style="width:100%">
                <asp:Label ID="lblFreightType" runat="server" Text="Freight Type: *" CssClass="label" />
            </td>
           <td  style="width:100%;">
                 <asp:DropDownList ID="ddlFreightType" AutoPostBack="true" runat="server" CssClass="dropmenu" OnSelectedIndexChanged="ddlFreightType_SelectedIndexChanged"
                    Width="160px" />
            </td>

         <td style="width:100%">
        <%--        <asp:RequiredFieldValidator ID="rfvFreightType" runat="server" ControlToValidate="ddlFreightType" ErrorMessage="Select the Freight Type" CssClass="label" />--%>
            </td>
       </tr>
       <asp:UpdatePanel ID="upFreightType" runat="server"  UpdateMode="Conditional" >
        <ContentTemplate>
             <tr  align="left">
                <td  style="width:100%;">
                <asp:Label ID="lblQuantity" runat="server" Text="Quantity :" CssClass="label" />
               </td>
            <td  style="width:100%;">
                <asp:TextBox ID="txtQuantity"  runat="server" CssClass="textBox" Text="1" Width="40px" />
                <input type="button" value='-' onclick='javascript:decrement(-1)' />
                <input type="button" value='+' onclick='javascript:increment(1)' />
             </td>
              <td style="width:100%;">
                 <asp:Label ID="lblQuantityPostFix" runat="server" />
            </td>
        </tr>
      <tr align="left">
           <td  style="width:100%;">
                <asp:Label ID="lblBasicAmount" runat="server" Text="Basic Amount:" Enabled="false" CssClass="label" />
            </td>
            <td colspan="2"  style="width:100%;">
                <asp:TextBox ID="txtBasicAmount" runat="server" CssClass="textBox" Width="160px" />
            </td>
       </tr>
      </ContentTemplate>
        <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlFreightType" EventName="SelectedIndexChanged" />
        </Triggers>
        </asp:UpdatePanel>