Distinct Items

Each item that is duplicated

List<String> duplicates = lst.GroupBy(x => x)
                             .Where(g => g.Count() > 1)
                             .Select(g => g.Key)
                             .ToList();

A list of all duplicates

var duplicates = lst.GroupBy(s => s)
    .SelectMany(grp => grp.Skip(1));


Another Way
  List<String> list = new List<String> { "6", "1", "2", "4", "6", "5", "1" };
    var q = from s in list
            group s by s into g
            where g.Count() > 1
            select g.First();

    foreach (var item in q)
    {
        Console.WriteLine(item);

    }

 var arrString = "Hello Ronda, I'm calling you Ronda. How are you Ronda.".Split(" ".ToCharArray());
        
 var duplicates = from wordCollection in arrString
                  group wordCollection by wordCollection   into g
                  where g.Count() > 1
                  select new { g.Key, Count = g.Count() } ;