Drag Drop from ASP.Net GridView prevent duplicate record without cancelling sortable plugin

I have the drag-and-drop working.

if you try to drag-and-drop a duplicate and get the error message, the drag-and-drop process stops working.

So is there any way to prevent duplicates and do a copy instead of move without cancelling the sortable plugin?

Here is my code:

<%@ Page Title="" Language="C#" MasterPageFile="~/EconomicInterest.Master" AutoEventWireup="true" CodeBehind="Testing.aspx.cs" Inherits="EconomicInterestWebApp.Testing" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajax" %>
 
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server"> 
    <link href="../css/PagerStyle.css" rel="stylesheet" type="text/css" />     
    <script type="text/javascript"> 
        $(function () {
            $(".drag_drop_grid").sortable({
                items: 'tr:not(tr:first-child)',
                cursor: 'crosshair',
                connectWith: '.drag_drop_grid',
                axis: 'y',
                dropOnEmpty: true,
                change: function (e, ui) {
                    var isExists = false;
                    if ($(this).attr("id") != ui.item.closest("table").attr("id")) {
                        $(this).find(".ItemId").each(function () {
                            if (!isExists) {
                                isExists = ui.item.find(".ItemId").html() == $(this).html();
                            }
                        });
                        if (isExists) {
                            $(".drag_drop_grid").sortable("cancel");
                            alert("Duplicate Item!");
                        } else {
                            $("<tr>" + ui.item.html() + "</tr>").insertAfter($('[id*=gvSource]').find("tbody").find('tr').eq($(ui.item).index() - 1));
                        }
                    }
                }
            });
            $("[id*=gvDest] tr:not(tr:first-child)").remove();
        }); 
    </script> 
    <asp:UpdatePanel ID="UpdatePanel1" runat="server"> 
        <ContentTemplate> 
            <asp:HiddenField ID="hv_isChanged" runat="server" Value="false" />
            <ajaxToolkit:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0" TabStripPlacement="Top" AutoPostBack="true"> 
                <ajaxToolkit:TabPanel ID="pnlTesting" runat="server" HeaderText="Testing"> 
                    <ContentTemplate> 
                        <asp:GridView ID="gvSource" runat="server" CssClass="drag_drop_grid GridSrc" AutoGenerateColumns="false">
                            <Columns>
                                <asp:BoundField DataField="ItemId" HeaderText="Id" ItemStyle-CssClass="ItemId" />
                                <asp:BoundField DataField="Item" HeaderText="Item" />
                                <asp:BoundField DataField="Price" HeaderText="Price" />
                            </Columns>
                        </asp:GridView>
                        <hr />
                        <asp:GridView ID="gvDest" runat="server" CssClass="drag_drop_grid GridDest" AutoGenerateColumns="false">
                            <Columns>
                                <asp:BoundField DataField="ItemId" HeaderText="Id" ItemStyle-CssClass="ItemId" />
                                <asp:BoundField DataField="Item" HeaderText="Item" />
                                <asp:BoundField DataField="Price" HeaderText="Price" />
                            </Columns>
                        </asp:GridView> 
                    </ContentTemplate> 
                </ajaxToolkit:TabPanel> 
                <ajax:TabPanel ID="pnlBlah" runat="server" HeaderText="Blah"> 
                    <ContentTemplate> 
                        <p>Testing</p> 
                    </ContentTemplate> 
                </ajax:TabPanel> 
            </ajaxToolkit:TabContainer> 
        </ContentTemplate> 
    </asp:UpdatePanel> 
</asp:Content>