How do I...Filter combobox values based on another combobox ?

i don't use data binding on combobox , my code to add combobox items is:

connection.Open();
OleDbCommand cmd = new OleDbCommand();
cmd.Connection = connection;
string query = "select * from client_details";
cmd.CommandText = query;
OleDbDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
country_txt.Items.Add(reader["country_name"].ToString());
customer_txt.Items.Add(reader["customer_name"].ToString());
}

i privent duplicates using this code for each combobox :
List<object> list = new List<object>();
{


foreach (object o in customer_txt.Items)
{
if (!list.Contains(o))
{
list.Add(o);
}
}
customer_txt.Items.Clear();
customer_txt.Items.AddRange(list.ToArray());