script type="text/javascript">
        var clicks = 0;
        function checkclick(obj) {
            clicks = clicks + 1;
            if (clicks>1) {
                alert("請勿重復點擊！");
                return false;
            }
            return true;
        }
    /script>

asp:button ID="btnSubmit" runat="server" text="查詢" OnClientClick="return checkclick(this);" OnClick="btnSubmit_Click" />