How to get duplicate items from a list using LINQ?

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

Note that this will return all duplicates, so if you only want to know which items are duplicated in the source list, you could apply Distinct to the resulting sequence or use the solution given by Mark Byers.

Here's another option:

var list = new List<string> { "6", "1", "2", "4", "6", "5", "1" };

var set = new HashSet<string>();
var duplicates = list.Where(x => !set.Add(x));

Here is one way to do it:

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

The GroupBy groups the elements that are the same together, and the Where filters out those that only appear once, leaving you with only the duplicates.