Eric White has a post on finding duplicates using LINQ.  Here is the sample:

  1: int[] listOfItems = new[] { 4, 2, 3, 1, 6, 4, 3 };
  2: 
  3: var duplicates = listOfItems
  4:     .GroupBy(i => i)
  5:     .Where(g => g.Count() > 1)
  6:     .Select(g => g.Key);
  7: 
  8: foreach (var d in duplicates) {
  9:     Console.WriteLine(d);
 10: }