insert statement

Hi,

In my application im using asp.net form view

<asp:SqlDataSource ID="dsCategories" runat="server" ConnectionString="<%$ ConnectionStrings:AAPDB %>"
        DeleteCommand="DELETE FROM [POCategories] WHERE [CategoryId] = @CategoryId"
        InsertCommand="INSERT INTO [POCategories] ([MainCategoryName], [CategoryName], [Description],[ClientId]) VALUES (@MainCategoryName, @CategoryName, @Description, @ClientId)"
        SelectCommand="SELECT [CategoryId],[MainCategoryName], [CategoryName], [Description] FROM [POCategories] WHERE [CategoryId] = @CategoryId"
        UpdateCommand="UPDATE [POCategories] SET [MainCategoryName]= @MainCategoryName, [CategoryName] = @CategoryName, [Description] = @Description WHERE [CategoryId] = @CategoryId"><SelectParameters><asp:ControlParameter Name="CategoryId" ControlID="lstCategories" PropertyName="SelectedValue" /></SelectParameters><DeleteParameters><asp:Parameter Name="CategoryId" Type="Int32" /></DeleteParameters><UpdateParameters><asp:Parameter Name="MainCategoryName" Type="String"/><asp:Parameter Name="CategoryName" Type="String" /><asp:Parameter Name="Description" Type="String" /><asp:Parameter Name="CategoryId" Type="Int32" /></UpdateParameters><InsertParameters><asp:Parameter Name="MainCategoryName" Type="String" /><asp:Parameter Name="CategoryName" Type="String" /><asp:Parameter Name="Description" Type="String" /><asp:Parameter Name="ClientId" Type="Int32" /></InsertParameters></asp:SqlDataSource>

  public bool DirectoryExists(string directory)
    {
        bool directoryExists;
        DropDownList clientid = (DropDownList)FormView1.FindControl("ddlClient");
        DropDownList maincategory = (DropDownList)FormView1.FindControl("ddlmaincategory");
        TextBox CategoryName = (TextBox)FormView1.FindControl("txtCategoryName");
        string path = @"ftp://a-argent.com/aargen000d/QPortal/Reports/Clients/" + clientid.SelectedItem.Text + "/" + maincategory.SelectedValue + "/" + CategoryName.Text + "/";
        FtpWebRequest request = (FtpWebRequest)FtpWebRequest.Create(path);
        request.Credentials = new NetworkCredential("aargen", "Zorro123");
        request.UsePassive = true;
        request.UseBinary = true;
        request.KeepAlive = false;
        request.Method = WebRequestMethods.Ftp.MakeDirectory;

        try
        {
            using (request.GetResponse())
            {
                directoryExists = true;
            }
        }
        catch (WebException)
        {
            directoryExists = false;
        }
        return directoryExists;
    }

 protected void FormView1_ItemInserted(object sender, System.Web.UI.WebControls.FormViewInsertedEventArgs e)
    { TextBox CategoryName = (TextBox)FormView1.FindControl("txtCategoryName");

         DirectoryExists(CategoryName.Text );}

output in databse

Cateid  maincate               subcategory   clientid
66	Business Operations	test 44	NULL	32
67	Business Operations	test 44	NULL	32

Here in subcategory names are duplicates .if  subcategory is there with same name  I don't  know  how to not insert  duplicate subcategory  but when I click on insert button it inserts .