Sys.InvalidOperationException: Two components with the same id autoCompleteExtenderPopupBehavior can't be added to the application in ASP.Net

<asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:TextBox ID="txtUser" runat="server" AutoPostBack="true"></asp:TextBox>
<asp:AutoCompleteExtender ID="extSearchMember" runat="server" ServicePath="~/services.asmx" TargetControlID="txtUser" ServiceMethod="GetMembers" MinimumPrefixLength="1" CompletionSetCount="25"/>
<asp:Button id="www" runat="server" CausesValidation="False" OnClick="www_Click"></asp:Button> 
</ContentTemplate>
</asp:UpdatePanel>

Some times i get the error

"Microsoft JScript runtime error: Sys.InvalidOperationException: Two components with the same id 'autoCompleteExtenderPopupBehavior' can't be added to the application."

at this time click events not working.. how to solve it