how to restrict selecting same value from dropdownlist asp.net

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ASP.NET

Gridview

drop-down

i have a gridview.it contains two empty text boxes,one dropdownlist control.for dropdownlist control data is taken from sql server.when i click on submit tha data in gridview(two textboxes,dropdownlis values)should be saved in sql server.my probelm when i click on submit,if same values selected in dropdownlist then an popup error must be displayed.
give the answer in asp using c#.



please help me.thanks in advance.

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false"
    OnRowDataBound="GridView1_RowDataBound" >

    <Columns>
        <asp:TemplateField HeaderText="name">
            <ItemTemplate>
                  <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="age">
            <ItemTemplate>
                  <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="sex">
            <ItemTemplate>
                <asp:DropDownList ID="ddlSex" runat="server">
                    <asp:ListItem> MALE </asp:ListItem>
                     <asp:ListItem> FEMALE </asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="seatno">
            <ItemTemplate>
                <asp:DropDownList ID="ddlSeatno" runat="server"/>
            </ItemTemplate>
        </asp:TemplateField>

    </Columns>
</asp:GridView>

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            BindGridView();
        }
        
    }

    private void BindGridView()
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("select seatno from busseats where busno='"+lblBusno.Text+"' and status='"+lblStatus.Text+"'", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        con.Close();
        GridView1.DataSource = ds;
        GridView1.DataBind();
    }
    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            con.Open();
            var ddl = (DropDownList)e.Row.FindControl("ddlSeatno");
            //int CountryId = Convert.ToInt32(e.Row.Cells[0].Text);
            SqlCommand cmd = new SqlCommand("select seatno from busseats where busno='" + lblBusno.Text + "' and status='" + lblStatus.Text + "'", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            con.Close();
            ddl.DataSource = ds;
            ddl.DataTextField = "seatno";
            //ddl.DataValueField = "StateID";
            ddl.DataBind();
            ddl.Items.Insert(0, new ListItem("--Select--", "0"));

        }
    }