<script>
    function rowClick(sender, args) {
        //if currently clicked item is already selected
        if (args.get_item().get_selected()) {
            //disable the auto postback feature
            sender.ClientSettings.EnablePostBackOnRowClick = false;
            setTimeout(function () {
                //bring back the feature on
                sender.ClientSettings.EnablePostBackOnRowClick = true;
            })
        }
    }
</script>

<telerik:RadGrid ID="RadGrid1" runat="server" AllowPaging="True" Width="800px" OnNeedDataSource="RadGrid1_NeedDataSource">
    <ClientSettings EnablePostBackOnRowClick="true">
        <ClientEvents OnRowClick="rowClick"/>