public static IEnumerable<T> FindDuplicates<T>(this IEnumerable<T> enumerable) {
  var hashset = new HashSet<T>();
  foreach ( var cur in enumerable ) {
    if ( !hashset.Add(cur) ) {
      yield return cur;
    }
  }
}

List<SomeType> list = GetTheList();
list
  .FindDuplicates()
  .ToList()
  .ForEach(x => x.State = "DUPLICATE");

List<obj> keys = new List<object>();

foreach (MyObject obj in myList)
{
    if (keys.Contains(obj.keyProperty))
        obj.state = "something indicating a duplicate here";
    else
        keys.add(obj.keyProperty)
}

IEnumerable<T> oldList;
IEnumerable<T> list;

foreach (var n in oldList.Intersect(list))
   n.State = "Duplicate";