eRKa

Накидал пример, получите все айдишники повторных комментариев, которые надо удалить.

static void Main(string[] args)
{
    var comments = new[]
    {
         new Comment { Id = 1, Text = "1" },
         new Comment { Id = 2, Text = "2" },
         new Comment { Id = 3, Text = "3" },
         new Comment { Id = 4, Text = "1" },
         new Comment { Id = 5, Text = "2" },
         new Comment { Id = 6, Text = "2" },
         new Comment { Id = 7, Text = "1" },
         new Comment { Id = 8, Text = "4" },
         new Comment { Id = 9, Text = "5" },
         new Comment { Id = 10, Text = "2" },
    };

    var result = comments
				.GroupBy(x => x.Text)
				.SelectMany(x => x.Select(y => y.Id).Skip(1))
				.ToArray();

    foreach (var id in result)
    {
         Console.WriteLine($"{id}");
    }
    Console.ReadKey();
}

class Comment
{
     public int Id { get; set; }
     public string Text { get; set; }
}