Design Pattern in C# to reduce Code Duplication looping through Class Lists

I have a rep with Media Items Each items can have several bookmarks and comments from different users.

When I come to update the repo I need to get all existing Comments and book marks and append the new comments and book marks, the code works, but i cant think of a clean way to do this generically that will reduce duplication, as more fields like this are planned

  public class MapValues
    {
        private readonly string _userName;
        private readonly Media _media;
        private readonly IMediaRepository _mediaRepo;

        public MapValues(string userName, Media media, IMediaRepository mediaRepo)
        {
            _userName = userName;
            _media = media;
            IMediaRepository = mediaRepo;
        }

        public async Task<Media> AppendExistingUserValues(Media updatedMedia, string userName)
        {
            var existingMedia = await _mediaRepo.GetByIdAsync(updatedMedia.Id);
            var existingBookmarks = GetBookmarksExcludingUser(userName, existingMedia);
            var existingComments = GetCommentsExcludingUser(userName, existingMedia);

            if (updatedMedia.Bookmarks.Any())
            {
                existingBookmarks.AddRange(updatedMedia.Bookmarks);
            }

            if (updatedMedia.Comments.Any())
            {
                existingComments.AddRange(updatedMedia.Comments);
            }

            updatedMedia.Bookmarks = existingBookmarks;
            updatedMedia.Comments = existingComments;
            return updatedMedia;
        }


        private List<MediaBookmarks> GetBookmarksExcludingUser(string userName, Media media)
        {
            var exisitingBookmarks = new List<MediaBookmarks>();
            if (media?.Bookmarks != null)
            {
                exisitingBookmarks.AddRange(media.Bookmarks
                    .Where(x => !string.Equals(x.Name, userName, StringComparison.CurrentCultureIgnoreCase)).ToList());
            }

            return exisitingBookmarks;
        }

        private List<MediaComments> GetCommentsExcludingUser(string userName, Media media)
        {
            var exisitingComments = new List<MediaComments>();
            if (media?.Bookmarks != null)
            {
                exisitingComments.AddRange(media.Comments
                    .Where(x => !string.Equals(x.Name, userName, StringComparison.CurrentCultureIgnoreCase)).ToList());
            }

            return exisitingComments;
        }
    }

    public class Media
    {
        public IEnumerable<MediaBookmarks> Bookmarks { get; set; }

        public IEnumerable<MediaComments> Comments { get; set; }
        public string Id { get; set; }
    }

    public class MediaBookmarks
    {
        public string Name { get; set; }
    }

    public class MediaComments
    {
        public string Name { get; set; }
    }