C# – Clicking checkbox causes onCheckedChanged to fire twice

I have a GridView with a TemplateField with a checkbox. My goal is to capture the onclick event using autopostback and setting a database flag. My only problem is that the event fire's twice. The first time The Checkbox (In the sender parameter) holds the clicked value so I set it based on the click. The second time the sender parameter has a checkbox that is always checked=false. I am happy to entertain suggestions on other approached to solving this problem but my goal is to set a database flag based on the user checking a checkbox. I am targeting .NET Framework 2.0.

Here is the associated code:

<div style="margin-left : 1em;margin-right:1em;">
    <asp:GridView ID="RouteGridView" runat="server" AllowPaging="True" 
            AutoGenerateColumns="False" CellPadding="4" DataKeyNames="ROUTE_NUMBER" 
            ForeColor="#333333" GridLines="None" style="width:100%;" 
        onselectedindexchanged="RouteGridView_SelectedIndexChanged" 
        AllowSorting="True" onpageindexchanging="RouteGridView_PageIndexChanging" 
        onsorting="RouteGridView_Sorting" >
            <Columns>
<%-- Column one --%>
<asp:TemplateField HeaderText="Route" SortExpression="ROUTE_NUMBER">
    <ItemTemplate>
        <asp:LinkButton ID="HyperLink1" runat="server" CommandName="Select" CommandArgument='<%#Eval("ROUTE_NUMBER")%>'  
                            Text='<%# Eval("ROUTE_NUMBER") %>' ></asp:LinkButton>
    </ItemTemplate>
    <ItemStyle HorizontalAlign="Center" />
</asp:TemplateField>

<%-- Column 2 this is where the problem CheckBox is--%>
<asp:TemplateField HeaderText="Read?" 
    SortExpression="READ_FLAG">
    <ItemTemplate>
        <asp:CheckBox ID="CheckBox1" runat="server" 
            OnCheckedChanged="ChangeReadFlag"  
            AutoPostBack="true"
            Checked='<%# (string)DataBinder.Eval(Container.DataItem, "READ_FLAG") == "1" %>' Enabled='<%# isSelectedRow(Container)  %>' />
    </ItemTemplate>
    <ItemStyle HorizontalAlign="Center" />
</asp:TemplateField>

<%-- more columns --%
<%-- more columns --%>
</Columns>
</asp:GridView>

Here is the event handler from the code behind:

protected void ChangeReadFlag(object sender, EventArgs e)
{
    if (RouteGridView.SelectedIndex != -1)
    {
        CheckBox cb = ((CheckBox)sender);
        DataKey key = RouteGridView.SelectedDataKey;

        //... do stuff here ...
    }
}