Disable asp:ImageButton during postback

Hi All,
My goal: Prevent users from submitting a form more than once.

I have googled and binged but can't seem to find a workable solution that disables an ImageButton after a clean validation, then does the postback. The closest I have come, with the code below, simply disables the ImageButton after a clean validation, but does not do the postback.
<asp:ImageButton ID="Submit" runat="server" ImageUrl="button.jpg" onclick="Submit_Click" onclientclick="if ( Page_ClientValidate() ) { this.disabled=true; } " usesubmitbehavior="false"/>

Any help/pointers would be greatly apprecaited!


-Randall



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