How to group a list?

Question

C++ / C#

Tell me how to group a list from a database using linq?
Here is the code where I connect and get the necessary data from the database, but there are repetitions and I need to group them somehow.
How to do it?

public static List<DataActor20MoviesInfo> GetDataActor20Movies()
        {
            using (var db = new LinqToDB.Data.DataConnection(LinqToDB.ProviderName.PostgreSQL, Config.SqlConnectionString))
            {
                var list = (from actor in db.GetTable<DVDrentalActor>()
                            join film in (db.GetTable<DVDrentalFilmActor>())
                            on actor.ID equals film.ActorID 
                            select new
                            {
                                ActorID = actor.ID,
                                FilmActorID = film.ActorID,
                                FirstName = actor.FirstName,
                                LastName = actor.LastName
                            }
                           )
                           .Select(s => new DataActor20MoviesInfo
                            {
                                ActorID = s.ActorID,
                                FilmActorID = s.FilmActorID,
                                FirstName = s.FirstName,
                                LastName = s.LastName
                           })
                           .ToList();

                return list;
            }
        }

public class DataActor20MoviesInfo
        {
            public int ActorID { get; set; }
            public int FilmActorID { get; set; }
            public string FirstName { get; set; }
            public string LastName { get; set; }
            public int Count { get; set; }
        }