using (var session = DocumentStoreHolder.Store.OpenSession())
{

    //This list is used to put in the database -- Careful!!!
    List<classA> List_01 = new List<classA>();

    //This list is from database, used to compare with the new coming data.
    //We will check with the list got from data.
    List<ClassA> List_02 = session.Query<classA>().ToList();

    //This uses the method Except to remove the duplicates between the new data
    //and the current list of matches in database.Return data is the new match
    //that is not existing in DB

     List_01 = List_02.Except(List_00, new IdComparer()).ToList();
     foreach (classA data in List_01)
     {
         //Why does it not store data in DB?!?!?!?!?
         session.Store(_match);
      }
     session.SaveChanges();
}