ASP.NET

using (Html.BeginForm("Login", "Default", new { ReturnUrl = ViewBag.ReturnUrl, data = Request.QueryString["data"] }, FormMethod.Post,
new
{
    id = "idtest1",
    onsubmit = "this.querySelectorAll('button').forEach(i => i.disabled = false)",
    @class = "form-signin",
    role = "form"
}))
{

}

<asp:DropDownList runat="server" ID="ddlCourse" DataTextField="Course"
                  DataValueField="Course" DataSourceID="SDSCourse" 
                  Width="260px" onChange="CheckNerveCenter"/>

<asp:CheckBox ID="chkNerveCentre" runat="server" Text="Nerve Centre" TextAlign="Left" />

Function CheckNerveCenter() As Object
    'If ID="ddlCourse" contains any word like "*Nurs*" then
    'uncheck ID="chkNerveCentre" and then disable ID="chkNerveCentre"
End Function

Error.
An error occurred while processing your request.

SELECT DISTINCT does not work the way I want.
SELECT DISTINCT works for the class column but not for name_event column
If "MyId" is e.g. CW and there are 4 lines there but name_event should only be 2 pcs, but it shows 4 pcs

Now display:
World Championships
French Champioships
World Championships
World Championships


It should be:
World Championships
French Champioships

What am I doing wrong?

<pre>        Dim MyDataAdapter As New SqlDataAdapter("SELECT DISTINCT class, nationality, name_event from statistics_results Where class = @class", MyConnection)

        MyDataAdapter.SelectCommand.Parameters.Add(New SqlParameter("@class", SqlDbType.VarChar, 10))
        MyDataAdapter.SelectCommand.Parameters("@class").Value = MyId

        Try
            Dim dt As New DataTable()
            MyDataAdapter.Fill(dt)

            Dim natlist = dt.AsEnumerable().[Select](Function(r) r("nationality").ToString())
            Dim natvalue As String = String.Join("- ", natlist)
            litNat.Text = natvalue

            Dim eventlist = dt.AsEnumerable().[Select](Function(r) r("name_event").ToString())
            Dim eventvalue As String = String.Join("<br>", eventlist)
            litEvent.Text = eventvalue

        Catch exc As SqlException

        Finally
            MyConnection.Close()
            MyConnection.Dispose()

        End Try