Prevent submission on Button Click if item appears more than once in Tagit TextBox in ASP.Net

Hi lingers,

Check this example. Now please take its reference and correct your code.

Database

For this example I have used of Northwind database that you can download using the link given below.

Download Northwind Database

HTML

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<link rel="stylesheet" href="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.24/themes/smoothness/jquery-ui.css" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.24/jquery-ui.min.js"></script>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/tag-it/2.0/css/jquery.tagit.min.css" />
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/tag-it/2.0/css/tagit.ui-zendesk.css" />
<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/tag-it/2.0/js/tag-it.min.js"></script>
<script type="text/javascript">
    $(function () {
        $.ajax({
            url: "<%=ResolveUrl("Default.aspx/GetCustomers") %>",
            data: "{}",
            dataType: "json",
            type: "POST",
            contentType: "application/json; charset=utf-8",
            success: function (data) {
                var sampleTags = [];
                $.each(data.d, function (i, item) {
                    sampleTags.push(item);
                });
                $(".myTags").tagit({
                    availableTags: sampleTags
                });
            },
            error: function (response) {
                alert(response.responseText);
            },
            failure: function (response) {
                alert(response.responseText);
            }
        });

        $(document).on('click', '[id*=btnSave]', function () {
            var ids = [];
            $.each($('[id*=GridView1] tr').find('.tagit-choice'), function () {
                ids.push($(this).find('input[type=hidden]').val());
            });
            $('[id*=hfCustomerIds]').val(ids.join(','));
        });
    });
</script>
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False">
    <Columns>
        <asp:BoundField DataField="ContactName" HeaderText="Name" ItemStyle-Width="100" />
        <asp:BoundField DataField="Country" HeaderText="Country" />
        <asp:TemplateField HeaderText="CustomerID">
            <ItemTemplate>
                <ul id="myTags" class="myTags">
                </ul>
                <asp:HiddenField ID="hfIds" runat="server" />
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>
<asp:HiddenField ID="hfCustomerIds" runat="server" />
<asp:Button ID="btnSave" Text="Save" runat="server" OnClick="OnSave" />

Namespaces

using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Web.Services;
using System.Linq;

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        GridView1.DataSource = GetData();
        GridView1.DataBind();
    }
}

protected void OnSave(object sender, EventArgs e)
{
    List<string> tags = hfCustomerIds.Value.Split(',').ToList();
    List<string> duplicates = tags.GroupBy(x => x).SelectMany(g => g.Skip(1)).ToList();
    if (duplicates.Count == 0)
    {
        // Do rest of task.
    }
}

[WebMethod]
public static string[] GetCustomers()
{
    List<string> customers = new List<string>();
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "SELECT [CustomerID] FROM [Customers]";
            cmd.Connection = conn;
            conn.Open();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    customers.Add(sdr["CustomerID"].ToString());
                }
            }
            conn.Close();
        }
    }

    return customers.ToArray();
}

private DataTable GetData()
{
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    string query = "SELECT TOP 5 * FROM Customers";
    using (SqlConnection con = new SqlConnection(conString))
    {
        SqlCommand cmd = new SqlCommand(query);
        using (SqlDataAdapter sda = new SqlDataAdapter())
        {
            cmd.Connection = con;
            sda.SelectCommand = cmd;
            using (DataTable dt = new DataTable())
            {
                sda.Fill(dt);

                return dt;
            }
        }
    }
}