var query = lst.GroupBy(x => x) .Where(g => g.Count() > 1) .Select(y => y.Key) .ToList();
var list = new List<string>(); list.GroupBy(n => n).Any(c => c.Count() > 1);
var list = new List<string>(); // Fill the list if(list.Count != list.Distinct().Count()) { // Duplicates exist }