Controls displaying twice after async postback

I have a update panel which has a textbox, label  in it

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional" >
             <ContentTemplate>
                            <td style="font-size:large;text-align:right">
                                <asp:Label ID="LblUserName" runat="server" Text="User Name" Font-Bold="true"></asp:Label>
                            </td>
                            <td style="text-align:center">
                          <asp:TextBox ID="TxtUserName" runat="server"  Width="200" TabIndex="4" onblur="textchange();"></asp:TextBox>
                            </td>
                            <td>
                                <asp:Label ID="LabelUsern" runat="server"></asp:Label>
                            </td>
                            </ContentTemplate>
                            <Triggers>
                                    <asp:AsyncPostBackTrigger ControlID="chkUSerIDcheck" />
                            </Triggers>

</asp:UpdatePanel>

The control  "chkUSerIDcheck" is a checkbox outside the Update panel.It has a code written in vb file.

After the async postback happens teh controls in the update panel display twice on the page and out of position

What could be the mistake here?