onclick跟onclentclick两个事件

onclick和onclentclick两个事件
我在做网页的时候有一个功能就是点一个按钮的时候有两个触发事件，它的效果是原来显示的现在影藏，影藏的那个就显示，可是我在点击后居然在几秒后又执行了二次操作，这个是怎么回事，求大神帮忙，下面是代码

<h2 class="title">
    <asp:UpdatePanel ID="up" runat="server" UpdateMode="Conditional">
        <ContentTemplate>
            <div>
                <table width="100%">
                    <tr>
                        <td id="tb_1" style="width: 55%">
                            <asp:Literal ID="lDescriptionBrief" runat="server" />                            
                              </td>
                              <td id="tb_4" style="width:55%; display:none">
                              <asp:TextBox ID="txtDescriptionBrief" runat="server" Width="48%" MaxLength="500"
                                        Height="35px" Style="font-size: 12px;" />
                              </td>
                            <td id="tb_2" style="width:25%">
                                <asp:HyperLink ID="HyperLink1" runat="server" />
                                <b style="font-size: 15px; padding: 12px">iGuide Catalogue #<%= Request["ItemID"] %></b>
                            </td>
                            <td id="tb_3" style="width: 20%">
                                <asp:PlaceHolder ID="phView" runat="server">